sounds like you have a corruption in winlogon.exe.
try rebooting the pc and constantly tappin f8, choose boot into safe mode,,if you can get into safe mode do a system restore.(if you dont know how to post back).....
the olny other thing to do is try a repair install boot from the xp cd (make sure bios is boot from cd rom,,,at startup tap "delete key) change boot device to cdrom..
wack ya cd in and restart boot from in.....it will go through normal setup rubbish,,,,,press r when promtped to do a repair install .not r to boot to recovery console...
this will thenn re-install xp over itself.leaving your data intact.
let me know if my answer ia too complex i will try my best too simplfy it.
sorry for any typos i have been up since 5 am.........
regards
danzil