Drive-by adware is not an issue with Firefox or Opera. You can use either one.
However, make sure you get rid of all icons for internet explorer when you install Firefox/Opera, and replace them all with Firefox/Opera icons in the exact same spots. They are located on the desktop, on the quick launch bar, and in the start menu. Also, disable the URL bar in Windows Explorer, so he doesn't "accidently" open IE. If he still does, kick his ass, because he's just being stubborn.
Also, after installing your new web browser, follow these steps.
1. Start Firefox or Opera. Close it.
2. Start IE. It will complain about not being the default browser. Click no when it asks if you want to make it the default browser, and make sure you uncheck the "always check on startup". At this point, you can get rid of all of your IE icons. Leave the icon for windows update, though.
3. Start Firefox or Opera again. This time, it should ask if it should be made default. Click yes, and make sure you check the "Always check on startup" check box.
Make sure you have a decent firewall installed as well, because that's a way for malicious trojans to appear. I'll give you a hint, it's not Norton and it's not McAfee. It's Kerio Personal Firewall 2.1.5. Later versions have a ton of bloat that you don't need, KPF 2.1.5 does everything you need, nothing you don't. And it's free too.
You can find it here: http://www.pricelesswarehome.org/2005/PL2005SECURITY.php
In order to figure out if an incoming or outgoing connection is good or bad, look at the connection box for a few seconds. If you do something like try to connect to AOL Instant Messenger, and you get the box popping up saying that the program AOL instant messenger wants to connect to the internet, allow it. Otherwise, deny it, especially if it's trying to latch on to the process lsass, system32, kernel32, or svchost.
If you want to go from a fresh install, before reformatting, burn the install files for both Firefox/Opera and Kerio both on a CD (along with other vital goodies). Install Windows with your internet connection UNPLUGGED. After installing KPF and Opera (along with all your drivers) plug in your internet and run Windows Update (and if you're running Windows XP, make sure you get Service Pack 2 with those updates) until it says no more critical updates found (it should take a few restarts).
After you got a clean install, get a free version of AVG Anti-virus from http://www.grisoft.com/doc/1 (requires the download link being sent to your email, but it's worth the hassle), and you're all set. Hell, after this kind of install, I doubt you even need Ad-aware or Spybot, but just in case, install Ad-Adware and Spybot.