Vssadmin Windows Vista (Also Available in Windows 7 and XP)

Command line tool for managing Shadow Copies in Windows:

Vssadmin Resize ShadowStorage increases or decreases the amount of disk space available to Shadow Copy (i.e. System Restore).  Handy if you find yourself running low on disk space and System Restore is the culprit.

Windows Vista:

Vssadmin Resize ShadowStorage /For=C: /On=C: /MaxSize=5GB

Windows 7:

Vssadmin Resize ShadowStorage /For=C: /On=C: /MaxSize=10%

Recognizes KB, MB, TB, PB, and EB for MaxSize.  Not specifying MaxSize will give Vssadmin unlimited disk usage.  More information on TechNet.