First of all, just let me state an opinion. It's my belief that the anti-virus companies are selling nothing but fear. While your PC can never be 100% impenetrable, some basic precautions and a little commonsense will do wonders.
1) The firewall in WinXP sucks a**. Download and install the free version of
http://www.zonelabs.com/store/content/company/products/znalm/freeDownload.jsp?lid=zadb_zadown">ZoneAlarm. ZoneAlarm will popup a dialog every time a program on your machine tries to access the internet or something on the internet tries to access your machine. You simply give permission for the programs you trust to access the internet and deny permission for everything else. If you don't know what a program does, you can click on a 'more info' button in the ZoneAlarm dialog and you'll be taken to a webpage that will tell you more about that particular program and whether or not you might want to trust it.
2) Next try
http://www.beemerworld.com/tips/servicesxp.htm">this page. Disable any unnecessary Windows services, particularly Windows Messenger (not to be confused with MSN Messenger). Not only will your machine be more secure, odds are it will appear to be faster too! This page is one of many, I just chose it as it ranked fairly high on a google search. Your mileage may vary.
3) Now run the
https://grc.com/x/ne.dll?bh0bkyd2">Shields Up! tests at Gibson Research Corporation. Steve Gibson is a bit of a self-aggrandizing blowhard, but he really does know his stuff. The various tests on this page will tell you how secure your machine is. Run all five tests: File Sharing, Common Ports, All Service Ports, Messenger Spam (you disabled this in step 2 above, right?) and Browser Headers. Each test explains itself and if there is cause for action, links to easy to understand directions.
4) Now, download and install Spybot Search & Destroy. Remember to check for updates and download them if necessary the first time you run the program. Then scan and eliminate any malware it finds. After any malware has been eliminated, then use its immunize feature. Download and install updates regularly. Scan regularly (once a month should be fine).
5) Download and install
http://seb.mozdev.org/firebird">Mozilla Firebird. Make it your primary web browser. Aside from the built-in popup blocking, you'll love the tabbed browsing (great for DU!) and other great features built-in. Also, since it is based on the Gecko rendering engine, it is arguably the world's most standards compliant web browser. That means, you get to see web pages the way the authors intended. Now, I do have to warn you that Firebird is a wonderful product. It is my primary browser and even on an old K6-II 500MHz machine with 64MB of memory and running Win98 (not even SE) it is very fast and responsive. However, it is still pre-release software and so it can be a little harder to deal with than IE. BTW: I recommend the beautiful SkyPilot theme for Firebird.
5a) If you absolutely, positively can't live without Internet Explorer, then download and install the
http://toolbar.google.com">Google Toolbar. Google searching built right into your browser's toolbar AND built-in pop up blocking. I forgot to mention above that Firebird includes Google (or any search engine you could want) built-into its address bar. However, if you check the various extensions for Firebird you will see there is an opensource Google toolbar for Firebird. This is handy as it provides all the functionality of Google's own toolbar.
6) Do away with Outlook Express. Download and install
http://www.mozilla.org/projects/thunderbird">Mozilla Thunderbird. Make it your default mail client. Another pre-release project from the Mozilla crew. This is everything a mail client should be; feature rich and bloat free. After just a few days, you'll never know how you got along without the built-in Bayesian spam filtering. I also commend the Skypilot theme for Thunderbird.
6a) If you absolutely, positively can't do without Outlook Express... Sorry, I know of no way to make Outlook Express more secure.
7) Now, use your Windows Update service and plug up Windows some more.
8) If you haven't already, reboot.
If you follow all of the above, and use a little common sense about indiscriminately clicking on things or downloading too-good-to-be-true software, then you should have no need for antivirus programs. In all my years of being connected to the internet (since 1994 if you're curious, and I do have broadband), I have never gotten a virus or worm. Some malware, but that was my fault. If you really need that little program to tell you the weather, might I suggest looking for a similar application on Freshmeat.net or Sourceforge.net first. Opensource software is your friend.
If you have any questions, feel free to e-mail or PM me. Hope this helps.