The speed of login to the domain can be limited by several factors.
Is the domain controller in the same site as the workstations? Domain controller authentication across a slow WAN link can take an extremely long time to complete.
There is a known and documented issue with Windows 2000 slow login that was corrected in SP 3. See here for details on the hotfix:
http://support.microsoft.com/kb/306850
In addition, is DNS functioning correctly on the systems? Incorrect DNS settings can cause issues with domain login.
XP differs from previous versions of Windows in that it uses DNS as it's primary name resolution method for finding domain controllers. Be sure that the Windows 2000 and XP systems point to the local DNS-name space server and not to an external DNS server.
Are there any errors showing in the event logs of either the local workstations or the domain controller?
How many GPO's are being processed by these systems having the issue?
You may experience extremely long delays (up to 5 minutes) when logging into domains using Windows XP Pro. This is caused by the asyncronous loading of networking during the boot up process. This speeds up the login process in a stand-alone workstation by allowing the user to log in with cached logon credentials before the network is fully ready.
To disable this "feature" and restore your domain logons to their normal speed, open the MMC and add the group policy snap-in. Under Computer Configuration-->Administrative Templates-->System-->Logon, change "Always wait for the network at computer startup and logon" to ENABLED.
This can be fed to clients via a group policy from a Windows 2000 server by upgrading the standard policy template with the XP policy template. Since this is an XP only command, non-XP systems will ignore it in a domain distributed group policy.
Windows 98 systems do not process Group Policy Objects; thus, they are seldom affected by issues with GPOs.