Try entering the BIOS and setting the time and date. Make no other changes, then exit for a re-start.
Does the problem persist? Try resetting you CMOS. Instructions vary slightly, most boards require the CMOS battery to be removed and the CMOS jumper moved to pins 2 & 3 for 10 seconds. Then, replace jumper on pins 1 & 2 and replace the battery. Power on, enter BIOS and set time and date. At this point your BIOS will be reset, and the system should work properly. If you are still geting the checksum error, you may need to replace your CMOS battery.
NOTE: The instructions for resetting the CMOS are for a P5B-D board. Your board may differ slightly, check your manual before doing this operation.
Once you have a stable load to the desktop, power-off and re-start to make sure that the changes were accepted and that the CMOS is saving and loading the BIOS correctly.
Hope this helps!
[SIZE=\"1\"]E6400 @ 3.4 , Noctua 120
7900GT @ 670/1680 , VF900
2 x 1GB Patriot 6400 @ 425
WD SATA 250GB
Neo HE 550
P180 / G15 / MX1000[/SIZE]