I believe Vista allocates 300mb for backups. Want to change that setting for whatever reason? Here we go
* Open an Admin Command Prompt (right click on cmd, and select run as Administrator)
* type 'vssadmin Resize ShadowStorage /For=[DRIVE]: /On=[DEST]: /MaxSize=[SIZE]MB'
* where [DRIVE] is your main drive
* where [DEST] is destination drive
* where [SIZE] is the size to change it to.