You can take control of how much diskspace is used by some of the Hidden file.
.. Restore points - Limit the abount of space they can take. memory serves me right it can set this space to 10% of drive, so for a 1 TB drive that is 100 Gigs. Each restore point takes 300 MBs, so over time this folder can become quite large. Limit it to say 3 to 5 gigs. Three giges would alow for 10 restore ponts, after twn the oldes one is push out.
.. Take control of your Virtual memory (page file). Windows typically sets this to 1.5 times your RAM so if you have 8 Gigs of ram this is about 12 Gigs. For an 8 GIG ram system I recommend setting BOTH Min and Max to the same value, 1024 MBs (for a 4 gig system try seting both to 2048 mbs). For an 8 gig Ram system this save approx 11 Gigs (5 Gigs for a 4 gig Ram system) WITH no adverse effects.
.. Hibernation sets a hidden file equal to 75-100% of Ram. If you do not need, or use hibernation disabling it will save about 6 -> 8 gigs of Disk space.
For:
.. Virual memory:
http://windows.microsoft.com/en-us/windows-vista/change-the-size-of-virtual-memory
.. Limit Restore points: http://mintywhite.com/windows-7/7maintenance/change-limit-system-restore-windows-7/
.. Disable Hibernation:
http://support.microsoft.com/kb/920730
The above three Items could account for/save upto about 100 Gigs.