I store all my data and the most of applications (those legally available in "portable" versions) on drive
D: and only use drive
C: for the Windows system and some heavily integrated applications like MS Office, Visual Studio, Adobe Reader, Flash Player etc. When I was using Windows XP, 50 GiB drive
C: was more than enough. Now, as I've mitigated to Windows 7, it hardly is.
Yesterday as I've checked, 7 GiBs were free on drive
C:. Then I've installed fresh Windows updates (which were just some tens of MiBs to download) and checked again: now there are only 2 GiBs free. Where have 5 GiBs gone?
PS: Don't be surprised my system installation takes actually that much: I've got Visual Studio and SQL Server with complete offline documentation library, but that doesn't explain where does free space disappear on simple Windows updates.
PPS: I use an augmented CCleaner version to clean my PC every day, so there are for sure no temporary internet files of recycle bin trash files taking the place.