If you're using Windows XP, before you do any changes mentioned in this thread, first go to your start menu and look for something called "System Restore" (probably under the Accessories -> System Tools menu). This will pop open a window asking if you want to restore your system, or create a restore point. Choose to create a restore point.

Then make your system changes. If you don't like them, go back and run System Restore. This time, choose to restore your system. You'll be presented with a calendar listing all of the dates upon which system restore points were created. Choose the day upon which you created the above system restore point. Your system will be restored to the way it was before the changes were made.