First try the use last configuration settings that worked ... do this by tapping 'F8' at startup and choose it when the menu allows you to .... if that doesn't work, here is another possibility ...
What usually helped me in this case **WARNING: may not have software entries, so you may have to reinstall some stuff ..
Start the Windows XP Recovery console and login to your windows
at the promt (should be "C:\WINDOWS")
type "cd system32\config"
type "rename default default.bak"
type "rename sam sam.bak"
type "rename security security.bak"
type "rename software software.bak"
type "rename system system.bak"
then type ..
copy "C:\WINDOWS\Repair\default"
copy "C:\WINDOWS\Repair\sam"
copy "C:\WINDOWS\Repair\security"
copy "C:\WINDOWS\Repair\software"
copy "C:\WINDOWS\Repair\system"
If this STILL doesn't work, reinstall windows, but make sure to choose RECOVER this windows installation
and hopefully you'll have a working OS