You can try a fix from the recovery console. If this doesn't work you probably need to do a repair install:
http://michaelstevenstech.com/XPrepairinstall.htm
If this computer came with XP pre-installed you need to follow the manufacturer's instructions for a recovery.
To try the Recovery Console:
Using either your XP CD or a boot disk:
http://support.microsoft.com/?kbid=310994
Enter thr Recovery Console:
http://www.kellys-korner-xp.com/win_xp_rec.htm
when promted type:
md TMP [Enter]
copy c:\windows\system32\config\system c:\windows\tmp\system.bak [Enter]
delete c:\windows\system32\config\system [Enter]
copy c:\windows\repair\system c:\windows\system32\config\system [Enter]
exit [Enter]
then run System Restore That file is your registry, and it's unrecoverable. The only solution is to use a default registry, simulating a reinstall (what those commands do), then run System Restore to get back your most recently working settings
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For a more comprehensive repair follow the following:
md tmp
copy C:\windows\system32\config\system C:\windows\tmp\system.bak
copy C:\windows\system32\config\software C:\windows\tmp\software.bak
copy C:\windows\system32\config\sam C:\windows\tmp\sam.bak
copy C:\windows\system32\config\security C:\windows\tmp\security.bak
copy C:\windows\system32\config\default C:\windows\tmp\default.bak
delete C:\windows\system32\config\system
delete C:\windows\system32\config\software
delete C:\windows\system32\config\Sam
delete C:\windows\system32\config\security
delete C:\windows\system32\config\default
copy C:\windows\repair\system C:\windows\system32\config\system
copy C:\windows\repair\software C:\windows\system32\config\software
copy C:\windows\repair\sam C:\windows\system32\config\sam
copy C:\windows\repair\security C:\windows\system32\config\security
copy C:\windows\repair\default C:\windows\system32\config\default
