Try running in safe mode. I've read where disk cleanup and defrag both can STILL be buggy, even in XP.
To start the computer in safe mode
You should print these instructions before continuing. They will not be available after you shut your computer down in step 2.
Click Start and then click Shut Down.
In the drop-down list of the Shut Down Windows dialog box, click Restart, and then click OK.
As your computer restarts but before Windows launches, press F8.
On a computer that is configured for booting to multiple operating systems, you can press F8 when the boot menu appears.
Use the arrow keys to highlight the appropriate safe mode option, and then press ENTER.