If you are not able to boot to desktop, then follow the below methods and check if it helps.
How to start your computer by using the Last Known Good Configuration feature in Windows XP: http://support.microsoft.com/kb/307852
A description of the Safe Mode Boot options in Windows XP: http://support.microsoft.com/kb/315222
If you are able to login to safe mode then I would suggest you to follow step 2:
Perform clean boot and check if it helps.
How to configure Windows XP to start in a "clean boot" state: http://support.microsoft.com/kb/310353
Note: Once you are done with the troubleshooting, perform the steps form “Steps to configure Windows to use a Normal startup state “to reset computer as usual.