If I may, I'd add: 95% of "viruses" aren't viruses at all, but are actually adware, junkware, worms, trojans, hijackers, and such. Antivirus alone won't protect you. You need a suite of software, which includes a good firewall. If you are using a broadband connection, which is connected all the time, you're a target. My suggestions to new computer users:
1. If you use broadband (DSL or cable), use a router with built-in NAT, even if you only have one computer .
2. Use a software firewall, even if the router has a hardware firewall. There are plenty of decent free firewalls around, but download from a TRUSTED and RELIABLE site - NOT "joesfreefiles.com"
3. Spyware protection is essential. Spywareguard, Spybot Search & Destroy (and Teatimer), Winpatrol to name a few. Don't get frustrated and turn off notifications in these programs! Every time one of them pops up a notice that "program X wants to do this or that," read it and decide what you want it to do. Don't just click "OK" and go on. You'll save yourself a lot of problems later. Install the program, read the documentation, learn how to use it and what id does (and doesn't do).
4. Reformat and re-install Windows isn't a FIX for most problems. I've been using computers since the days of DOS 2.2, and I've NEVER reformatted a hard drive and re-installed the OS. That's just a cover-up for the real problem: YOU didn't properly protect your system from the "bad guys".
5. Your HOSTS file is your friend. Use a custom HOSTS file, keep it up to date, and write-protect it.
6. If you don't have a clue what I'm writing about, turn off your games, music, DVD movies, etc. Go to Google, search on any or all of the things I just wrote about, and LEARN. Or, please, disconnect your computer from the internet so I don't catch all the crapware you've made yourself a magnet for.