Install an antivirus program or better yet, an internet security suite; keep it up to date using the auto update feature; and configure it to scan instant messages, as well as every file you open. I can't guarantee that you will never get a virus, but you will have to work pretty hard to get one.
Of course if we all went out and brought an Apple or used Linux and Firefox, then we would be immune to viruses, at least until Apple, Linux, and Firefox capture enough market share to make writing viruses for those OS's and applications worth the effort. I guess that's one of the really good things about marketing a product that relatively few people want or use or even know or care about.
Yep - already way ahead of you. I'm just being a grouch about all the viruses, worms, and hackers out there - I've got better things to do with my time than to constantly download & install Win patches and updates (but I do it anyway). Then there seems to be 2 or 3 Norton AV updates everyday. Then there's Zone Alarm, MS Office, Norton Internet Security, bios upgrades, hardware driver updates, new toys, industry software programs, website maintenance, email, and on and on. Hell, if I could get away from this dang computer for an hour or 2 a day, maybe I could make some money, lol.