This topic has come up a few times in the lab recently, so I decided to write a guide on it. I’ll omit the Linux advertisement, but if you are going to (or have to) use windows for something, here are some good practices. All of these tools are (at the time of this writing) free to the public for home use.
Back up important stuff. This goes for everyone, actually. (Never mind the bit about me not practicing what I preach…)
Use avast! antivirus. It is fairly unobtrusive and has its virus definitions updated really often. If you don’t like avast!, people alternatively use AVG (although they’re getting better and better at hiding the free version). I believe that both programs come set up automatically, but you should make sure that your software is A. updating itself regularly and B. actually performing scans at some point.
Anti-spyware stuff tends to be good too. Antivirus will catch some of this, and they’re better and better at it lately, but I still prefer to use a cocktail of anti-bad-stuff on my windows machines. I recommend using a combination of Spybot Search & Destroy and Ad-Aware. NOTE: it seems that, unfortunately, Ad-Aware has gone the way of ZoneAlarm. The link to the free version exists but is broken. Gross. If you can find a free version of it please post the link. Otherwise, I must unfortunately recommend that readers do not install Ad-Aware by jumping through the hoop of their stupid “complete one offer” scam. Be sure to update your anti-spyware after you install it. Spybot in particular has an “Immunize” function in addition to scanning. This is perhaps the biggest motivator for installing spybot. Blocking spyware entirely is awesome, and makes sure you waste less time removing stuff over and over.
Use Firefox. (Chrome seems cool too, but I don’t have any tips for securing Chrome, or much knowledge about it in general). Lock down your Firefox using NoScript and AdBlock Plus. NOTE: After installing these two plugins, websites are going to look a little…different. Particularly NoScript. There will be a little ‘S’ in the bottom right hand corner of your Firefox now that you will often have to click and tell it to ‘allow’ scripts from certain trusted websites. This can be annoying at first, but is worth the added security, especially if you tell it to remember your choice and always trust Google.com, slashdot.org, sourceforge.net, Edwardslab.sdsu.edu or whatever. Basically, any site you know isn’t trying to do something to your computer you can trust, but beware that the plugin is only as good as your decisions are.
I also recommend WebOfTrust for finding out what websites are shady or not in the first place.
I hope these links help you secure your Windows install. If you have any questions about using the aforementioned software, feel free to ask in the comments or email me and I’ll do my best to help!