I just bought an MSI K9N SLI motherboard but when I turn on my computer I receive this error at startup:

CMOS Checksum Bad
******* Warning Unknown Processor Revision *******
The processor(s) installed in your system are of an unknown revision.
Please contact your BIOS vendor for appropriate updates.
Fatal Error... System Halted.

I have checked on MSI's website and it says that my AMD 5200 processor is compatible with the motherboard.

Code:

Processor: AMD 5200
Motherboard: MSI K9N SLI
GPU: Nvidia 8800gts
PSU: 400w
Hard Drive 1: Hitachi 160gb IDE
Hard Drive 2: Western Digital 320gb SATA

First off there are a few versions of the AMD 5200 as seen here:

MSI -- MICRO-STAR INT'L CO., LTD.

The error probably means you need a newer BIOS.

The manual should have info on how you can flash from a hot key combo. If you need help with this let us know and we will download the manual.

Here is a guy with a similar issue:

Bios Updating MSI K9N Platinum.. - Overclock.net - Overclocking.net

Sounds like all you need is to update your bios.