You can disable it through the System Properties. Since I'm not on my computer right now, I can't tell you exactly how to do it, but here's a simple outline. Open up System Properties (right-click on MyComputer, then 'properties'), then on one of the tabs there's a bunch of checkboxes. The last check box will disable System Restore.
Personally, I wouldn't disable it altogether. You have the ability to lessen the amount of disk space, and modify the frequency of saved restore points via the System Properties tabs also. I set mine to the minimum of 200MB.
-CamaroGuy