I found this in a few different places around the 'net - hopefully it's helpful:

Clear System Restore Points for Performance

You keep your system as clean as possible, clearing your Temp Folder, and Temporary Internet files. But what about other space hogs?

What about System Restore Points? By default, Windows XP creates a restore point after installing Windows XP, once every 10 hours that Windows XP is running, or every 24 hours. It also creates a restore point when you install a new program, or install an update to Windows XP. And you can manually create restore points at any time. You can end up with a number of restore point files, which can be large.

So how do you clean them out? First, make sure your system is stable. There will be a brief period when you won't have a restore point.

• Click Start, click All Programs, click Accessories, click System Tools, and then click System Restore.
• Click to add a check mark beside Turn off System Restore on all Drives, and click Apply.
• When you are warned that all existing Restore Points will be deleted, click Yes to continue.
• All system restore points are deleted. Now you should manually create a restore point.
• Click Start, click All Programs, click Accessories, click System Tools, and then click System Restore.
• Click Create a Restore Point, and then click Next.
• Name your restore point. ( I use the date as well as a descriptive term such as "After Restore Point Deletion.")