Well Win7/Vista have an "restore" funcion called ShadowStorage, its turned on by default.
Win 7 Installation uses at start up to 19 GB (Ultimate) and after that the Storage is going to raise and you need to turn it off.
Use Google and search for: "How to resize Shadowstorage"
Well you cant turn it off, but you cant limit it at 900 MB (lowest).
just do this if you dont want to use Google,
run - type cmd.exe - run as admin.
type:
vssadmin help
vssadmin resize shadowstorage /For=C: /On=C: /Size=900MB.
Well, thats only you checked the logical point.