Working on a system recently, I noticed that its 20 GB primary partition was low on disc space. There was nothing immediately obvious as to the cause, so I broke out the windirstat utility. This gives a visual representation of the filesystem, using different sized blocks to identify large files, and color codes each file to show the data type. This great utility immediately pointed out some large files with the .msp extension that were taking up 5GB of space on the disk.
A quick google of the .msp file extension showed them to be cached Windows Installer files. I grabbed a copy of the Windows Installer cleanup utility (MSIZAP.EXE) and ran it from the command line, using the G switch.
The G switch specifies to clean up orphaned files only. Within seconds, the msizap utility had burned 4 GB of useless files off the disk. For a total investment of 15 minutes of time, I had solved the problem completely.