Here's a link to Microsoft website: http://windows.microsoft.com/en-US/windows-vista/System...
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.
Go to System Restore Wizard, click Start, and then click Help and Support. Click Performance and Maintenance, click Using System Restore to undo changes, and then click Run the System Restore Wizard.
If you want to undo an earlier restoration, click Restore my computer to an earlier time in the Welcome to System Restore screen, and then select the relevant restore operation restore point listed in the System Restore Wizard.