I worked on a PC that had no anti virus, FIREWALL or spyware/ad ware remover when i scanned the drive it had a minimum of 500 viruses , mostly Trojan horses. when i scanned the HARD drive and put it back in the machine the memory stoped working.

Someone bought the parts and built the PC. Here are some of the specifications

windows XP professional(not SURE if it was service pack 2 or 1. I am not even sure if it had any service packs installed)

Biostar M6VLR mainboard check out the link: http://www.boisterous.com/subtotals.asp?model=M6VLR

20 gig hard drive

ONE 256 MB memory card

one optical drive, LG  CD R, RW/ DVD ROM

What i want to know is this can viruses cause memory to go bad? If the answer is yes, how can you prevent this from happening?Are you talking about the RAM or the hard drive here?
Actually in either case the answer is no, viruses cannot damage hardware.
But a virus might corrupt the file system on a hard drive so it needs to be wiped before it will function again.What external forces will cause memory to go bad and leave all other parts of the computer in working order.RAM goes bad all the time.
It can be caused by a low quality PSU, a POWER spike or static electricity for example and sometimes RAM just go bad for no apparent reason.
It is one of the most delicate computer components.