Normally each mobo has a 3-pin jumper for initialise the Bios to the factory setting. Consult the mobo hand book.
It is usually jumpered with say pin 1 and 2 (could be pin 2 and 3). By removing the jumper and plugging into the other combination of pin 2 and 3 ( could be pin 1 & 2) for say 5 seconds and then back to the original pin 1 & 2 will cancel the user's setting and revert to the factory setting.
Removing the battery in the mobo also does the same trick.