Give this a shot:
From the Microsoft newsgroup:
To solve the problem where Disk Cleanup fails to complete the "compress
old files" option, run Regedit, and navigate to
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\Explorer\Vo
lumeCaches\Compress Old Files
Save this key in an exported .reg file (eg, compressoldfiles.reg) so
that you can restore it if necessary. Remove all the keys under
"Compress old files" to get this cleanup program to run to completion.
Same info different description
here