Did you ever heard about the WINSXS folder on your system? If not, it is now a good time to check the size of it. A few GB of disk space are used for a folder nobody knows why it is there at all.
The same situation applied to me, but after reading a few sites and blog entries I do know that it is important. It is storing all needed components Windows 7 needs during its life time. The idea is to hard link them from any place - so only one location is needed and no duplicates are created. Well, that is an awesome idea, but what Windows 7 is not doing - cleaning up! After you have installed SP1 for Windows 7 - some of these components are not needed any more - but the remain in the folder - the only reason is - if you would uninstall the SP1 the superseeded components can be reactivated.
I decided there is no need for that behaviour and using the tool DISM on the command line - all the superseeded components get deleted. Only "downside": you can not uninstall SP1 from this point onwards
The command I used to reclaim my disk space after updating my Windows 7 to SP1 with Windows Update
dism /Online /Cleanup-Image /spsuperseded