Many people have had this problem. OFP player Robbel recently reported this problem to Codemasters support. They suggested, for starters, that he
OFP veteran Hardrock recently experienced this error on his system. Turns out that on his PC's RAM memory modules was faulty. He used Microsoft's Windows Memory Diagnostic utility to confirm the RAM problem. Purchasing a new RAM module indeed resolved the problem for Hardrock.
OFP player SirWili says make sure that your system is completely clean of viruses. After discovering his system was infected, he removed the viruses and OFP no longer reported checksum errors.
After you perform the above preliminary steps, make sure that your system's BIOS program, chipset (especially the VIA chipset), graphics, sound and mouse drivers are up-to-date.
Been there? Done that? Some players found that the solution was to copy the original file (e.g., 1985.PBO, SOUND.PBO and others) from their OFP installation CD into the appropriate OFP subdirectory and retry installing the patch. Some people succeeded this way on the first try, others succeed only after retrying this 5 and even 15 times over. Go figure!
Other players have done even stranger things to succeed in installing or upgrading OFP. For example, some CD burner utilities, such as EZ-CD Creator, NeroBurn and especially CloneCD, have been known to cause problems when installing, upgrading and playing OFP. They may even incorrectly overlay your system's ASPI layer, according to some reports. If true, this could explain the CRC/checksum errors you are getting.
After going through all of the above and still not having any luck, OFP player Chris finally got things to work by changing his BIOS program's RAM speed setting from 133 to 100, rebooting and only then installing and upgrading OFP. Worked like a charm, says Chris. Just remember to reset the RAM speed back to 133 after you're done.