Have you thoroughly checked your machine for malware, including running some of the online virus scanners?
Try downloading SpyBot Search and destroy
and run that.
Do the same with Hijack This, you can post the log here if you like.
How to reset security settings back to the defaults
Some general things to try.
Try Safe Mode (Do you have this problem in Safe Mode?).
Try running ChkDsk to check your drive for errors. Right-click your Drive icon/ Properties/ Tools/ Error Checking. Select both boxes.
Run the System File Checker program from the Run Box by typing.....Sfc /Scannow in it and have your XP CD handy.
HOW TO: Verify Unsigned Device Drivers in Windows XP
If they don't work you could try repairing windows by running it over itself. You will lose all your windows updates but your files will be untouched.
How to Perform an In-Place Upgrade (Reinstallation) of Windows XP (Q315341)
If the repair install fails to fix the problem, and it may not as by design it carries over settings from one install to the other, try a clean install after saving your valuable data.
HOW TO: Partition and Format a Hard Disk in Windows XP (Q313348)
Read the paragraph,
"How to Partition and Format Your Hard Disk by Using Windows XP Setup."
For more help (shows enlargeable screenshots) go here.