I recently have helped several friends deal with viruses on Windows. My general solution is clean install and then set them up with AV/Firewall. I put together a little guide to help people out:
Best ways not to get a virus in Windows:
#1) Run up-to-date antivirus software. If you don't have any you can get one free at www.comodo.com
#2) Run a firewall. If you have Windows XP, Service Pack 2 and higher have one built-in. Also, you can get an advanced firewall for free at www.comodo.com
(same link as the free antivirus)
#3) Don't click to run e-mail attachments even if they're from people you trust. Viruses often disguise themselves as e-mails from people you trust or use someone's address book to send themselves to other people. This is the whole "Trojan Horse" concept where they need your naive help by clickin on all your attachments to take over your computer. If you do want to look at e-mail attachments, save them first to your HD and scan them for viruses. It also helps if you have known file extensions not be hidden so you don't run the wrong type of file. FWIW, most pictures are jpg, png, gif, or bmp. Don't open anything with the following extensions: exe, vbs, com, bat. And be quite cautious of any unknown extensions.
4) Don't click on popup ads or visit sites that ask you to install extra unknown software to view the site. Be cautious of any browser extensions or ActiveX controls you are asked to install.
5) Don't reply to SPAM or fill out forms with personal details on sites you don't trust. That's just asking for more SPAM or e-mails loaded with viruses in them.
6) Don't install catchy little programs like free screensavers, smiley tools, etc. Whatever you do, if you're browsing and a browser popup window says "you're infected with a virus, click here to run a scan" or "download this activex to optimize your computer" don't do it - Most of those fake virus scanner pop-ups actually install some form of malware (spyware, trojan, worms, viruses, etc) on your computer. Even if the program itself is not deliberately malicious (and quite a few of them are by including SpyWare that tracks you online), these programs often have connect to the internet and have security holes that allow another attack vector for viruses and worms to get on your computer.
In general, use common sense, be proactive in running defensive software, be paranoid of installing anything on your computer, and be smart enough to realize there's almost nothing free on the internet and anything that sounds to good to be true probably is -- there's a lot of snake oil out there. Finally, if you have room on another hard drive in your computer, consider making a backup or a copy of your uninfected HD for safety's sake. Once you get a computer infection in Windows, it's often very hard to remove and sometimes a clean install or reinstall of a backup is the only way to get your computer bug-free.