If you have them, then simply use the Recovery disc to format the disc, then reinstall the old operating system and all programs back to it's factory state.. If you don't have those Recovery Discs, then try contacting the manufacturer to get them.. One problem: Many computer manufacturers no longer support Windows Me systems.. It may not be possible to obtain the discs from the manufacturer.
In order to proceed, you'll need to let us know more about what items you still have in your possession.
Hope this helps.
Several years ago I bought a Thinkpad A21 with Windows ME installed. I replaced ME with XP. Now of course the computer is maxed out and unable to perform. I have bought a newer laptop and want to know if and how I can reinstall the ME system so I can give it to my granddaugter.