On an NTFS partition, Disk Cleanup can compress old files
to save space. But calculating the savings and performing
the compression often take a long time, and on some systems,
Disk Cleanup hangs during the process. If that happens, or if
you don't care to wait, use this Registry tweak to disable the
compression: Delete the key
HKEY_ LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\ CurrentVersion\Explorer\VolumeCaches\Compress Old Files.