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.