WinSXS (Side by Side) can take 10 gigs or more over time. Same with system restore points.
The most recent Flash, Java, and dotnet patches are 50 megs or more each.
If you have an app that downloads the dotnet 1.0 to 3.5 it can take 2 gigs more and then there are the service packs for that that are 300 megs.