The problem with missing or corrupted files (in system32\config folder)
can be solved two ways. In both cases you need a bootable CD (XP
installation CD is good for the purpose)
or another PC to which you can mount your HDD.
1) Scan your HDD for errors. The simple tool is windows's 'chkdsk' with
parametrs /f /r. This may take time but at least you'd be sure HDD
errors would be fixed.
2) After (or without) scanning, try to restore to previous state of the
system. If your system restore function disabled skip this part and go
to the step 3.
3) As a good advice restore your system with restore option that can be
run from windows XP installation CD.

As for using another PC... well, mount your HDD, get an access to
'System Volume Information' folder by giving rights to Aministrators
group.
There you can find folders named '_restore{547...ets .In it you'll find
several folders named 'RP010' or smth like that. You need to open
snapshot folder and rename files _REGISTRY_MACHINE_SOFTWARE &
_REGISTRY_MACHINE_SYSTEM into 'software' & 'system'. Then copy those
files into c:\windows\system32\config replacing files there.

In some cases you may need to restore _REGISTRY_USER_NTUSER_S-...etc
This file rename into NTUSER.DAT & copy into your user's profile folder
(For example c:\documents and settings\administrator)
The bigger the size of the file in a row of files named
_REGISTRY_USER_NTUSER that means the file is yours. (Other files are for
other profiles).