If Windows Update is enabled (which you can disable unless forced by a policy, be it local or domain) and will download whenever it feels like it. Updates will only be installed at the designated time in Windows XP, unless dictated by the aforementioned policy.
I belive Vista and 7 will immediately install updates that will not require a reboot, which is a policy option in XP. I have Windows 7 and here is my log:
2009-11-16 19:21:32:700 948 c34 DnldMgr *************
2009-11-16 19:21:32:700 948 c34 DnldMgr ** START ** DnldMgr: Downloading updates [CallerId = AutomaticUpdates]
2009-11-16 19:21:32:700 948 c34 DnldMgr *********
2009-11-16 19:21:32:700 948 c34 DnldMgr * Call ID = {3E10C45B-67A4-49F9-8211-42B877A7A90A}
2009-11-16 19:21:32:700 948 c34 DnldMgr * Priority = 2, Interactive = 0, Owner is system = 1, Explicit proxy = 0, Proxy session id = -1, ServiceId = {7971F918-A847-4430-9279-4A52D1EFE18D}
2009-11-16 19:21:32:700 948 c34 DnldMgr * Updates to download = 1
2009-11-16 19:21:32:700 948 c34 Agent * Title = Definition Update for Windows Defender - KB915597 (Definition 1.69.995.0)
2009-11-16 19:21:32:700 948 c34 Agent * UpdateId = {D8B08FA9-A5D8-4511-9735-CCCD4F72703D}.100
2009-11-16 19:21:32:700 948 c34 Agent * Bundles 1 updates:
2009-11-16 19:21:32:700 948 c34 Agent * {7FC67E83-4BD2-490F-8D1C-2E1CDAD9358D}.100
2009-11-16 19:21:32:700 948 c34 DnldMgr *********** DnldMgr: Regulation Refresh [Svc: {7971F918-A847-4430-9279-4A52D1EFE18D}] ***********
2009-11-16 19:21:32:700 948 c34 DnldMgr Contacting regulation server for 1 updates.
2009-11-16 19:21:32:700 948 c34 Misc Validating signature for C:\Windows\SoftwareDistribution\WuRedir\7971F918-A847-4430-9279-4A52D1EFE18D\wuredir.cab:
2009-11-16 19:21:32:716 948 c34 Misc Microsoft signed: Yes
2009-11-16 19:21:32:716 948 c34 PT URL for regulation server found in server config.
2009-11-16 19:21:32:716 948 c34 DnldMgr Regulation server path: https://www.update.microsoft.com/v6/UpdateRegulationService/UpdateRegulation.asmx.
2009-11-16 19:21:33:168 948 c34 DnldMgr * Regulation call complete. 0x00000000
2009-11-16 19:21:33:168 948 c34 DnldMgr *********** DnldMgr: New download job [UpdateId = {7FC67E83-4BD2-490F-8D1C-2E1CDAD9358D}.100] ***********
2009-11-16 19:21:35:742 948 c34 DnldMgr * BITS job initialized, JobId = {9838008B-B150-4DBE-862F-EDFAA681BB75}
2009-11-16 19:21:35:820 948 c34 DnldMgr * Downloading from http://au.download.windowsupdate.com/msdownload/update/software/defu/2009/11/mpas-d_f7b8ee1a6a1de4bf94e3da3db817ba38b91bfd66.exe to C:\Windows\SoftwareDistribution\Download\132fbe39f239519d94f61049b180c3da\f7b8ee1a6a1de4bf94e3da3db817ba38b91bfd66 (full file).
2009-11-16 19:21:36:038 948 c34 Agent *********
2009-11-16 19:21:36:038 948 c34 Agent ** END ** Agent: Downloading updates [CallerId = AutomaticUpdates]
2009-11-16 19:21:36:038 948 c34 Agent *************
2009-11-16 19:21:36:038 948 cc4 AU AU checked download status and it changed: Downloading is paused
2009-11-16 19:21:37:645 948 c34 Report CWERReporter finishing event handling. (00000000)
2009-11-16 19:21:45:664 948 cc4 AU AU checked download status and it changed: Downloading is not paused
2009-11-16 19:21:45:664 948 cc4 AU AU setting pending client directive to 'Download Progress'
2009-11-16 19:21:47:645 948 ae4 AU WARNING: AU found no suitable session to launch client in
2009-11-16 19:22:01:669 948 cec DnldMgr BITS job {9838008B-B150-4DBE-862F-EDFAA681BB75} completed successfully
2009-11-16 19:22:01:732 948 cec Misc Validating signature for C:\Windows\SoftwareDistribution\Download\132fbe39f239519d94f61049b180c3da\f7b8ee1a6a1de4bf94e3da3db817ba38b91bfd66:
2009-11-16 19:22:01:747 948 cec Misc Microsoft signed: Yes
2009-11-16 19:22:01:747 948 cec DnldMgr Download job bytes total = 1491328, bytes transferred = 1491328
2009-11-16 19:22:01:747 948 cec DnldMgr *********** DnldMgr: New download job [UpdateId = {7FC67E83-4BD2-490F-8D1C-2E1CDAD9358D}.100] ***********
2009-11-16 19:22:01:763 948 cec DnldMgr * All files for update were already downloaded and are valid.
2009-11-16 19:22:01:763 948 cc4 AU >>## RESUMED ## AU: Download update [UpdateId = {D8B08FA9-A5D8-4511-9735-CCCD4F72703D}, succeeded]
2009-11-16 19:22:01:763 948 cc4 AU #########
2009-11-16 19:22:01:763 948 cc4 AU ## END ## AU: Download updates
2009-11-16 19:22:01:763 948 cc4 AU #############
2009-11-16 19:22:01:763 948 cc4 AU Setting AU scheduled install time to 2009-11-17 09:00:00
2009-11-16 19:22:01:763 948 cc4 AU Successfully wrote event for AU health state:0
2009-11-16 19:22:01:763 948 cc4 AU Auto-approving update for install, updateId = {D8B08FA9-A5D8-4511-9735-CCCD4F72703D}.100, ForUx=0, IsOwnerUx=0, HasDeadline=0, IsMinor=1
2009-11-16 19:22:01:763 948 cc4 AU Auto-approved 1 update(s) for install (NOT for Ux), installType=2
2009-11-16 19:22:01:763 948 cc4 AU #############
2009-11-16 19:22:01:763 948 cc4 AU ## START ## AU: Install updates
2009-11-16 19:22:01:763 948 cc4 AU #########
2009-11-16 19:22:01:763 948 cc4 AU # Initiating minor updates install
2009-11-16 19:22:01:763 948 cc4 AU # Approved updates = 1
2009-11-16 19:22:01:763 948 cc4 AU <<## SUBMITTED ## AU: Install updates / installing updates [CallId = {36B107CF-111F-4BDA-A35B-85F25F593F04}]
2009-11-16 19:22:01:763 948 d24 Agent *************
2009-11-16 19:22:01:763 948 d24 Agent ** START ** Agent: Installing updates [CallerId = AutomaticUpdates]
2009-11-16 19:22:01:763 948 d24 Agent *********
2009-11-16 19:22:01:763 948 d24 Agent * Updates to install = 1
2009-11-16 19:22:01:763 948 cc4 AU Successfully wrote event for AU health state:0
2009-11-16 19:22:01:763 948 d24 Agent * Title = Definition Update for Windows Defender - KB915597 (Definition 1.69.995.0)
2009-11-16 19:22:01:778 948 d24 Agent * UpdateId = {D8B08FA9-A5D8-4511-9735-CCCD4F72703D}.100
2009-11-16 19:22:01:778 948 d24 Agent * Bundles 1 updates:
2009-11-16 19:22:01:778 948 d24 Agent * {7FC67E83-4BD2-490F-8D1C-2E1CDAD9358D}.100
2009-11-16 19:22:06:770 948 c34 Report REPORT EVENT: {B090A937-A9CB-4C16-B627-B4EF6C76BA46} 2009-11-16 19:22:01:763-0600 1 162 101 {D8B08FA9-A5D8-4511-9735-CCCD4F72703D} 100 0 AutomaticUpdates Success Content Download Download succeeded.
2009-11-16 19:22:06:770 948 c34 Report REPORT EVENT: {3A444B3C-576B-4F8F-BDDF-8886778B2F69} 2009-11-16 19:22:01:763-0600 1 188 102 {00000000-0000-0000-0000-000000000000} 0 0 AutomaticUpdates Success Content Install Installation Ready: The following updates are downloaded and ready for installation. This computer is currently scheduled to install these updates on Tuesday, November 17, 2009 at 3:00 AM: - Definition Update for Windows Defender - KB915597 (Definition 1.69.995.0)
2009-11-16 19:22:06:770 948 c34 Report CWERReporter finishing event handling. (00000000)
2009-11-16 19:22:22:979 948 d24 DnldMgr Preparing update for install, updateId = {7FC67E83-4BD2-490F-8D1C-2E1CDAD9358D}.100.
2009-11-16 19:22:22:994 3936 f84 Misc =========== Logging initialized (build: 7.3.7600.16385, tz: -0600) ===========
2009-11-16 19:22:22:994 3936 f84 Misc = Process: C:\Windows\system32\wuauclt.exe
2009-11-16 19:22:22:994 3936 f84 Misc = Module: C:\Windows\system32\wuaueng.dll
2009-11-16 19:22:22:994 3936 f84 Handler :::::::::::::
2009-11-16 19:22:22:994 3936 f84 Handler :: START :: Handler: Command Line Install
2009-11-16 19:22:22:994 3936 f84 Handler :::::::::
2009-11-16 19:22:22:994 3936 f84 Handler : Updates to install = 1
2009-11-16 19:22:29:375 3936 f84 Handler : Command line install completed. Return code = 0x00000000, Result = Succeeded, Reboot required = false
2009-11-16 19:22:29:375 3936 f84 Handler :::::::::
2009-11-16 19:22:29:375 3936 f84 Handler :: END :: Handler: Command Line Install
2009-11-16 19:22:29:375 3936 f84 Handler :::::::::::::
2009-11-16 19:22:29:406 948 cc4 AU >>## RESUMED ## AU: Installing update [UpdateId = {D8B08FA9-A5D8-4511-9735-CCCD4F72703D}, succeeded]
2009-11-16 19:22:29:437 948 d24 Agent *********
2009-11-16 19:22:29:437 948 cc4 AU Install call completed.
2009-11-16 19:22:29:437 948 d24 Agent ** END ** Agent: Installing updates [CallerId = AutomaticUpdates]
2009-11-16 19:22:29:437 948 cc4 AU # WARNING: Install call completed, reboot required = No, error = 0x00000000
2009-11-16 19:22:29:437 948 d24 Agent *************
2009-11-16 19:22:29:437 948 cc4 AU #########
2009-11-16 19:22:29:437 948 cc4 AU ## END ## AU: Installing updates [CallId = {36B107CF-111F-4BDA-A35B-85F25F593F04}]
2009-11-16 19:22:29:437 948 cc4 AU #############
2009-11-16 19:22:29:437 948 cc4 AU Install complete for all calls, reboot NOT needed