hmm, if the H/D is filled with viruses. then I am pretty sure, they killed the BIOS. replacing BIOS chip isnt that easy, as availability matters, when you open the motherboard, you'll see the BIOS chip yourself, as replacing it isnt a problem, pull it out and push new one in. BUT, it will be like blank book, so you have to do the Floppy Drive Method to flash it with new data.
at this point, I can only suggest one method (not guaranteed) as there could be 2 possibilities
1: BIOS chip is dead.
2: BIOS data is completely erased.
case 1 is fairly understood, case 2 = use the floppy drive BIOS flash method, it may work. see the motherboard manual, and go to their website and download the BIOS on floppy.
before doing that, as well confirm BIOS is faulty, by hearing the beeps (the beep codes will be in the manual).
IF, in any unfortunate case, the system is OEM. then OEM's don't give that much of Support other than sending them and getting the system repaired. (AKA you are at their control), if the system is old enough, then OEM wont help as well, you will be left with only 1 solution. send this back to customer with a spade to dig the grave (so ask him if you are invited on the funeral of the PC)