At an absolute minimum
At an absolute minimum, you need some kind of AV program, a basic firewall, and to get as far away from Internet Explorer as you possibly can. The firewall that comes with XP is perfectly fine, and the free AV programs are probably just as good, maybe better in some ways, than the large suites. Think of that like the equivalent of getting the oil changed in your car every so often and maybe rotating the tires. There's a lot more you SHOULD be doing, but that is the absolute bare minimum you can get away with.
As I already stated, there are no magic bullet solutions. My list is a pretty bare bones essentials guide as it is. It will only mitigate the risk of malware and other issues, it's not a fool-proof system. Maybe it's time to consider that looking to only do the absolute minimum is what caused your need to post here in the first place, and that if you had been a little more diligent, things might have been different. When it comes to security, you get out of it what you put in. My list already IS the minimum you should be doing. It's the equivalent of locking your doors and windows at night.
Just a possibly interesting bit of trivia, but the most common failure of home alarm systems, is people forgetting to turn them on.