Here's a link to Microsoft website:http://windows.microsoft.com/en-US/windows-vista/System-Restore-frequently-asked-questions
Click on 3rd point "Can I undo the changes System Restore makes" and it says:
Yes. Every time you use System Restore, a restore point is created before proceeding, so you can undo the changes if they don't fix your problem. If you use System Restore when the computer is in safe mode, you cannot undo the restore operation. However, you can run System Restore again and choose a different restore point if one exists.
To undo the changes System Restore makes
Open System Restore by clicking the Start button , clicking All Programs, clicking Accessories, clicking System Tools, and then clicking System Restore. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
Click Undo System Restore, and then click Next.
Review your choices, and then click Finish.