Do you get tired of those warnings about updating your anti-virus software? Or maybe you just don't want to spend more money for another piece of software. Well I've got good news and bad news.
First, the bad news: If you keep ignoring those warnings you'll probably pay more in the long run than if you had taken the time and the money to prevent yourself from getting some viruses and adware/spyware. Depending on how bad your system gets gucked up (that's a technical term) it can cost you hundreds of dollars to have someone clean it up.
Now the good news. It doesn't have to cost a lot to protect your Windows computer. There are several anti-virus programs that are free for home use. The one I use is called Avast Home Edition. "ClamAV" used to be another, but as I was preparing this I found out it is no longer available for Windows. AVG AntiVirus is another that I have used in the past. (Note: The two links in this paragraph were good at the time of this writing. You may have to hunt for them if they change.)
You'll also want to have good software to protect from Spyware and Adware. You may get all in one software if you go with the well known, more expensive ones like Norton and McAfee. If you're cheap frugal like me, you can get freeware programs like "Adaware" and "Spybot". To find others, do a search on "free spyware removal" and "free adware removal". Then compare.
Now the BEST news. Use Linux on your computer like I do and you don't have to worry about viruses. I've been using Linux for over 9 years now, have never installed any anti-virus software, and have never been bothered by any viruses. I worked at one of the major airlines in their control center where they had about 300 workstations using Linux. They never had any anti-virus software installed either. Theoretically Linux viruses exist and can infect a computer, but I've never heard of a real one that did any major trouble like the thousands of viruses for Windows and Mac OS. So yes, I'm plugging Linux again. ;-)