Main bios checksum error

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Depends on the severity of the checksum errors, if the PC can eventually successfully boot then in many cases a replacement of the CMOS battery can solve the problem. Once the battery is replaced boot into the BIOS and Load Optimized Defaults or Defaults then save.

If the problem continues then perform a Clear CMOS procedure - most PC's offer a 'Jumper'; see - http://www.youtube.com/watch?v=rdHH9KrceR0

If both fail to solve the problem then more than likely the CMOS is corrupt and unrecoverable so schedule an RMA to replace the MOBO or CMOS EPROM chip. You 'can' attempt to update or reflash the BIOS but never do that procedure through the OS and instead through the BIOS itself - note with a corrupted BIOS the risk is very high that you'll brick (ruin) the MOBO.