If you really need to do it and you have your stuff backup.
(1) Stick WinME installation CD into CD drive.
(2) Switch on PC, PRESS DEL key when you see something on screen, do it quickly, this will get you into Bios program
(3) Look around Bios and find Bootup Sequence (order) and set CD-ROM as first bootup device.
(4) Save change and quit Bios.
(5) Restart PC, now PC will bootup from WinME CD.
(6) Just follow on screen instruction.
Reformat will wipe eveything off, so only do it if it is necessary and you have your own files backup.