Unable to Log On To Windows XP

put your copy of xp in and boot from cd choose install then repair make sure you have the product key handy cuz you will need it. doing it this way you will not lose any vital info you may have. otherwise, reinstall windows clean from the same disk.

Couple of questions 1st. 1-Are you able to get into safe mode? 2-Can you Ctrl+Alt+Del at the welcome screen and then log in that way? 3-Have you tried using the left shift key when windows is starting up? 4-what version of DirectX and Lavasoft are you running?
If #1, It sounds like you have a software issue, go into safe mode and uninstall the program you think is causing the problem. Try to reboot and see what happens. Alternatively, you can do system restore to bring you to the point you were at before the problem occured.
If #4 I have encountered this problem running DirectX 8.0 and 9.0b. I would recc updating to the latest version. I would first do a system restore to some point when the computer was working good.
Can do system restore by start->Run->restore
then select rstrui.exe in the window that opens.

This sounds like a virus to me. This is going to be a tough one to crack.

From the same registry location you mentioned make sure that the entry for shell=Explorer.exe

To change the password you'll need Offline NT Password Recovery and Registry Editor, CD burner and since your online I'll assume you have a PC to connect to the Internet and work from. Offline NT Password Recovery and Registry Editor will work to fix the problems. Just follow the instructions very closely. Make a bootable CD using the download from the following site.

http://home.eunet.no/~pnordahl/ntpasswd/

Its a bit advanced but using the registry editor included with Offline NT Password Recovery you can fix the auto reboot problem with the following registry edit;

Use the help feature to learn how to navigate through the registry.

Registry Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CrashControl
Modify/Create the Value Data Type(s) and Value Name(s) as detailed below.
Data Type: Dword // Value Name: AutoReboot
Setting for Value Data:
0 = AutoReboot Disabled
1 = AutoReboot Enabled

Try to boot into safe mode.

Once you can get into the system you'll need to start the virus removal processes.

Disconnect from the internet.

Empty all but needed startup options using MSCONFIG, uncheck as much as possible.

Check each startup item using Google or some other search engine to see what the process really does. This may be the best way to find out what virus you have.

See what processes are running using Taskmanager.

Check in the windows and windows\system32 folders, show details and list the files by date. Newer exe and dll files may be the guilty parties. Again search for the filenames online.

Have you tried altering your BIOS boot set-up? When you reboot, access the BIOS set-up. Then go to the boot tab and then change it to the drive that contains a workable OS. Disable any other hard drive configurations that interferes with normal boot process, like disable the floppy and the CD-ROM drives for now. If you have any other HDD like in an array set-up, make sure that this is where your OS is located and should be enabled or if not, then disable it for now. You are left with one drive where the OS will boot from. Then save the new configuration, exit and reboot. When the BIOS boot up it will seek the drive that contains a clean OS, barring any virus or anything like that, you should be able to boot up to your welcome screen and then to your desktop. The loop process is there because the BIOS detects another drive in the system that allows it to seek an erroneous or corrupt OS. Happened to me but not with lavasoft, though.