Its pretty easy:
1. Patch your computer www.windowsupdate.com
2. Keep your antivirus software up to date
3. Download a firewall ie. zone alarm, its fee
4. Download adaware and spybot.
Its simple. Your computer SHOULD automatically download and update both windowsupdate files and antivirus files.
Problem is, your average MS patch opens up more holes than it closes. For the folks who get viruses, the better solution is to go back to using Windows 98, which does pretty much all the same things, is as or more stable, runs faster, and is less vulnerable to attacks. Win98+software firewall+virus scanner is all one needs, no patching or updates.
You can also get a freeware program that prompts you every time something tries to write to your registry.