System Restore Point
I recently configured an old laptop with a clean Windows XP installation. I started with the OEM CD, removed unnecessary OEM software that was hogging system resources, updated to Service Pack 2, installed all the latest patches, and installed a recent antivirus system. The system was running very smoothly, and I was very proud of my achievement. Over the weekend, the system was given to its new owner, who promptly screwed up the network configuration. It took me a few minutes to think of it, but I asked the new owner to do a system restore. The only problem was that I did not set a restore point, so the owner had to return to one of the restore points that was set automatically during my configuration. So the system is no longer pristine.
Moral of the story: when configuring a new Windows system for someone, make sure the last thing you do is to create a restore point under the System Restore utility.