Amazingly some people have been running their computer for years and have never scanned it for spyware or adaware. If you have windows popping up, applications starting or running that you did not install or maybe your homepage is now set to something you did not want, you are infected with spyware.
Surfing online or even bundled in some downloads can infest your computer with garbage that you did not ask for. In some cases you may have worms or trojans that collect your personal info or keystrokes and send it off to someone next time you get online. Some hacker may now have your bank account user name and password without you knowing.
Adaware or spyware will also slow your computer down. I had one case where the users computer was running at a very slow crawl due to so many programs running in the background. A build up of garbage collected over a few years and their computer was running at 98 percent just to keep up. A thorough scan and cleaning restored the computer to like new condition.
As a computer tech I used to carry my preferred software on a memory stick or CD and use it to run scans. I found it worth the price many times over to buy a good spyware remover. After trying almost every free one out there I found they did not do a complete job. You will soon discover this too if you decide to go the free route. Download two or three free programs and I bet each one of them will find stuff the others did not.
A professional program like XoftSpySe will do a complete job the first time. It is also the preferred software for most computer techs. Free programs do not have the funds to spend on keeping their definitions updated like the paid versions.
Once you decide on and install your software you will want to do a full scan for the first time. This may take a while. You will probably be shocked the first time you run a scan on the number of files that do not belong on your computer. After your first scan is complete you will want to re boot your computer and I am willing to bet you will notice a huge increase in performance and speed.
Scanning your system once a week or so will keep it clean and running smooth. You will also want to update and scan with what ever anti virus program is installed on your computer.