By default Windows Vista allocates 15% of your hard drive to storing System Restore points - which turns out to be a couple dozen gigabytes on a large drive. The user-friendly Windows XP slider bar which sets System Restore space has disappeared on Vista; instead a little command line action's required to reset this percentage.
To recover some of that System Restore disk space, start up Vista's command line as an administrator, and run vssadmin list shadowstorage to see the current allocation. To change it, use:
vssadmin resize shadowstorage /on=[drive]: /for=[drive]: /maxsize=[size]
That is, to limit System Restore to 2 gigabytes on the C: drive, use:
vssadmin resize shadowstorage /on=C: /for=C: /maxsize=2GB
Of course, limiting System Restore's drive footprint also limits how far back it can store points, so do so carefully. Here's more on rolling back Windows with System Restore.
Windows Vista, Reduce System Restore Disk Usage, Tips