If you fun SCANREG in the Msconfig, your computer will create a registry backup every day your computer is booted or left on, and keep them for 5 days. This means you can revert to a previous configuration up to 5 days ago. You can use this if your IRQs get messed up, if a program corrupts the registry, in some cases if your computer won't boot into Windows, or a number of other problems. Any hardware or software you installed or uninstalled since the day the registry you restore was created will be lost.
Here's how you restore your registry in Windows XP:

Go to Start - Run, and type SCANREG and hit OK or Enter.

You will need to use your cursor keys and Enter, as your mouse will not function in a text box.

Select Start, It will ask if you wish to backup or view backups. Select View Backups and hit Enter.

Scroll up and down with your cursor keys until you find the date of the registry you wish to restore. You can do this again if you choose the wrong one. Hit Enter and it will restore your registry to the previous state. The registry for that date was created on the first boot of the day or shortly after midnight.