As storage has gotten more expansive, Microsoft has gotten very good at unnecessarially using up this extra space. One of the biggest storage hogs is System Restore, which was rather easy to modify in Windows XP. In Windows Vista, there is no easy way to change the amount of space that System Restore uses, so Vista is likely keeping the restore point from the time you wiped your butt with a new roll of TP.
In Windows Vista, the storage problem has gotten even worse, because Vista now has found a way to bloat even System Restore. The new feature is called Volume Shadow Copy Service (VSS), which enhances System Restore by making backup copies of deleted or changed files. Most of these "shadow copies" cannot be deleted: they just take up space for no reason whatsoever.
Fortunately, there is a command line utility that will allow you change the amount of space Windows Vista uses for Restore Points and Shadow Copies, and is called the vssadmin.exe (Volume Shadow Copy Service Admin). Read more at OCModShop.