You can limit the size of the Shadowstorage capacity (this is where the System restore copies are saved). I think it's default size is around 15% of your HDD and for some with small drives or not much room this can be too much. I limit mine to around 6GB as this means I have at least one or two Restore's saved (a system restore size is typically 300MB).
To do this you need to look in your start menu and find 'Command Prompt'.
Then you need to right-click on this and choose 'run as administrator', you will then see a black box open on the desktop. In the black box type this word for word including spaces, it has to be exact otherwise it will not work:

vssadmin Resize ShadowStorage /For=C: /On=C: /Maxsize=6000MB

Then hit enter and if you've typed it correctly you'll see dialogue telling you the shadowstorage has been resized correctly. If you have more than one hard drive and you have the Vista install on D or E change the above letters accordingly. Also you can change the size, if you want to use a 4GB size then type 4000MB, as long as it's 300MB or more you'll get System restore copies saved.

Just to add, you can 'clear' your 'old' saved copies too... If you find 'Disk Clean-up' in the start menu, click on it and then hit 'more options'. Here you'll find at the bottom of the page an option to clear old system restores and shadow copies. It will always save at least one so don't worry about not having a copy at all. Remember you can always go and make another system restore yourself via the control panel.

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