One of the most under appreciated tools in Windows is the System Restore tool. Microsoft introduced this tool in the much hated Windows ME release.
It's helpful for removing viruses, resetting a machine after a coworker changes things, or fixing things that I screw up while experimenting with applications, drivers, and settings.
Periodically, the tool takes a "snapshot" of critical Windows system files, like the registry. It stores a certain number of these snapshots (depending on how much disk space you allow) and lets you reset Windows to an earlier point in time when you need to. It takes these snapshots on a time schedule, or when Windows updates, and some sometimes when new applications are added.
The user can also tell the system to create a snapshot whenever they like. This is a good thing to do before making system changes.
When someone tells me their system suddenly started acting weird, one of the first things I'll do (after other basic troubleshooting steps) is run a System Restore to get the machine back into the condition it was a week or a month earlier. This usually fixes the problem.
Unlike reinstalling Windows, System Restore affects only Windows files. It will not make changes to or damage your data.
After you run System Restore, you may need to reinstall applications you had installed after the machine took the snapshot you just restored to. But it's possible those applications were causing problems anyway.
In Windows XP, you can get to the tool by going to Start | Programs | Accessories | System Tools | System Restore.
In Vista or Windows 7, just hit the Start button, and then type "System Restore" in the search box.
Familiarize yourself with the tool if it's new to you. You may find it quite useful one day.