As a computer repair professional one of the most frequent problems I deal with is Virus, Spyware, and Adware infestation. With the abundance of free software available to prevent and/or fix these problems it amazes me to see it as often as I do.
Any one with an internet connection can get infected, and fortunately any one with an internet connection can get free, high quality antivirus and anti-spyware protection. Just go to your favorite search engine and search for free antivirus software. You will quickly get dozen of good results such as Avira, AVG, or Avast, which are three of my personal favorites. These products will install and run in the background to help protect your computer from infection and can run a full virus scan on your system at any time. You can also find free one time system scans such as House Call from Trend Micro. These can give you extra piece of mind if you think an infection made it past the antivirus product you have installed on your computer.
Free Anti-Spyware and Adware Removal software is even more abundant. Your search results will pull up dozens of available software to download and install for free. I personally use and recommend Spybot from Safer Networking to keep spyware and adware from installing behind the scenes while you surf the net. Malwarebytes and Ad-Aware make good reinforcements if you think any spyware or adware made it around Spybot.
There is one draw back to the free security software world. An increasing amount of spyware and adware is being packaged as Anti-Spyware to fool consumers in to installing a malicious product. If you are not very experienced with computer security you should stick with products recommended by a trusted source.