If you find 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:

Boot into DOS in Windows 98 by hitting F8 before Windows boots and choosing Command Prompt. Or in Windows ME, use your boot floppy disk.

Type SCANREG /RESTORE and hit return. It will give you a choice of registries to restore. Choose one, hit enter, and it will tell you to reboot. That's it!