Easiest way to ruin a PC running XP

Just kinda curious what is the easiest way to fuck up someones computer? Im not exaclty a pro at this sort of thing but some jackass (which i also happen to know in person) has been sending crap to my email which my antivirus sez is harmful (i opened the first one on another computer (R.I.P), but he keeps sending stuff). What I would like to know is where he gets this stuff? And, is there a smart way to retaliate or should I just let him be?

(note: id prefer to be able to return the favor, but ill stick to whatever is a more sound decision)

Unless he's a weenie who thinks its funny to send viruses to you, it's more likely that he's been infected with a worm that is trying to propagate to everyone in his address book. Tell him to sort it out. The internet is pretty sick of people whe can't secure themselves and get turned into spam zombies.