A 2003 system is about 5 years overdue of scrapping. I would carry it to a recycling center and get a $400 replacement box that will run circles around it.
Mine still plays modern games a decent settings. Nothing made to tax the system, but things like COD4 or Company of Heroes will do just fine. Don't diss it.

There are a number of virii that are designed to damage the BIOS. It's not all that hard. Just reverse engineer the BIOS update software for a common motherboard manufacturer and set it to run silently...even if it doesn't complete (you shut down the computer), it's still waxed the BIOS.
I recommend setting him up with FireFox and an extension called NoScript (and IE Tab if he wants, but that will circumvent NoScript). FireFox is incompatible with ActiveX, by far the most common method of delivering virii into a computer running IE. NoScript will automatically kill all Java, JavaScript, Flash, redirects, iFrames, ect. until the user specifically deems the site trustable (permanently or temporarily).
Just as a nice little touch, AdBlock Plus! will kill off just about every advertisement known to man. I always forget they even exist until someone complains about them.
EDIT: In the case of possible RAM damage, I suggest a bootable disk program called MemTest86+. It'll slap your RAM around like a schoolgirl until it cries or proves itself worthy.