When you run Disk Cleanup (Start / \[All\] Programs / Accessories / System Tools / Disk Cleanup), you are offered to Compress old files. If you never choose this option, or Disk Cleanup frequently hangs, delete the Compress old files registry sub-key:
1. Copy / Paste the following to a DeleteOldFiles.reg file:
\[-HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\VolumeCaches\Compress old files\]
2. Merge the DeleteOldFiles.reg file with your registry or run regedit /s DeleteOldFiles.reg.
NOTE: See How do I run Disk Cleanup in a batch?