I had a similar problem with an MSI board and the I845 chipset and P4 1.6 I tried all the things that are mentioned in the posts. reflashed bios, changed battery, etc, etc. I even went to the trouble of getting a replacement motherboard. What the problem ended up being was when I upgraded the equipment I used a preinstalled NT harddrive. The device drivers should be generic and such execpt for Network cards and such but it caused the system to act wonky. If I powered off and started back up boom there would be the bios checksum error. If I left the system up and modified the bios and saved changes everything would be fine. But cycle the power and voila instant checksum. I reinstalled the operating system with a clean install and no more problems. Weird as it may sound it did not like it. So if you tried the easy way for upgrading your computer and just modifying the drivers you had too for a new board then it may be it.
Harry.
All Spelling and Gramatical errors are on purpose for testing the readers abilities.