You need to protect your PC from the hundreds of new computer viruses that are created every year. I used to write about my own experiences with various antivirus software, but antivirus software changes so often that whatever I wrote was obsolete by the time anyone read it. It may take you awhile to find the best antivirus product for your needs and budget, but it will be worth it in the long run. For current comparisons of several major antivirus packages, see Virus Bulletin and AV Comparitives and ICSA Labs, which regularly publish unbiased comparative tests, reviews, and certifications of major antivirus products like the following:
AntiVir, AVG, and Avast! offer commercial versions and versions that are free for personal use. The other products listed above are only available as commercial packages. All products listed above allow you to download and use a free trial version of their software. At the end of the trial period (usually 30 days), the software stops working unless you buy it. The free antivirus solutions give you free updates for the life of the product. The commercial antivirus solutions generally give you free updates for a specified period of time (usually one year). At the end of that time, you have to pay a (usually yearly) subscription fee in order to continue getting updates.
Over the years, I have used several free and commericial antivirus products, including AntiVir, Avast!, AVG, Inoculan Antivirus, InoculateIT, McAfee Viruscan, and Norton Antivirus. I've been using the free version of AVG AntiVirus on my computers since February, 2002, but I'm always looking for either a better free antivirus product, or a commercial one that is so good that I would be willing to pay for it.