Within the same machine model, Dell had shipped different variant version of motherboards.  I suspected that the BIOS 2.6.6 is not compatible for your board. The 745 is very old machine, after many years, the only reason someone would try to update its BIOS is for a CPU upgrade.  With the current BIOS 2.6.4, I think you can upgrade to a 2.40 GHz Q6600 and attempt with a BSEL mod to bump its clock to 3.0 GHz. Just to give you some idea.  The result is vary.  Also requesting Mods to remove your tags.
I have a Dell Optiplex 745 with Windows XP Pro Service Pack 3 with four gigs of RAM. I am trying to update it to the latest BIOS update 2.6.6 on your website. I keep getting this error message Program Payload Error. I was able to update the BIOS before this one fine. I think it is 2.6.4.  The . Thanks for any help you can give me. [IMAGE]
Dell Optiplex 745 BIOS update error message Program Payload Error