1. Chances are your machine is completely uninfected with anything. There are no Mac viruses, but there IS one bit of malware that could be gotten by trying to download a "video codec" that claims to allow you to watch free porn. I wouldn't (obviously) put this past immature 20-something-year-olds, but they'd have REALLY had to have been on the hunt for this ONE SPECIFIC MALWARE TRICK and worked REALLY fast to get it on your Mac, so again I think the odds are against it (though that would explain why they "can't remember" the site they went to).
2. Nothing has infected your Mac BIOS, mainly because the Mac doesn't have BIOS to infect (but also because no trojan/malware/virus exists for the Mac that can affect the deeper levels of the system).
3. You know, this is exactly WHY there's a "Guest Account" available in Leopard and Snow Leopard, though I guess that may not have stopped them in this case ... anyway, next time, use the "Guest Account" option, it's safe and disposable.