Yeah David's right in saying this, as it could be the problem quite easily, if for example you've done a overhaul.
If your stuck on how to do it, try these steps. (Look on step 2 first as this will be more probable to work faster):
1. You could just take out the battery to the bios chip which stores the flash memory and it'll reset itself. Its one of those round ones like what you use in watches, and leave it out for about an hour or so, while computers turned off completly from power supply. Make sure when you put it back, you follow polarity too.
2. Some motherboards have something called a CMOS memory cleaning header (ABIT boards usually do).
It should be near the battery, yet not always right near it and underneath should be printed "CMOS", and it's got a jumper on it (like what you use to set Master, Slave on a hard drive) and it should be set to 3 for normal operation (the end one on the left out of 3 settings)
To clear it, set the jumper to space 1 (the end one on the right) and leave it for a few minutes. Then set this back to 3 for normal operation again, and test
Hope this helps
