Windows do not delete compressed files through the disk cleanup utility,
windows compresses files that have not been accessed for a period of time.
http://support.microsoft.com/?kbid=315246
<quote>
Compress Old Files - Windows can compress files that you have not used
recently. Compressing files saves disk space, but you can still use the
files. No files are deleted. Because files are compressed at different
rates, the displayed amount of disk space that you will gain is approximate.
An Options button permits you to specify the number of days to wait before
Disk Cleanup compresses an unused file.
</quote>