pete6666 :
how to reformat acer Aspire M3640 Aspire Desktop
Power the unit on.
Immediately hold down the ALT key and tap F10.
This will boot into the recovery console. Follow the steps from there to complete a full sytem recovery, taking the unit back to factory settings.
Once that has finished, dont forget to open add/remove programs [programs and features] and remove a lot of the rubbish that Acer preinstall. Half of it you dont need!