A lot of the above (and in the security link) is great info but well beyond the scope of your "average" computer user. If I know someone who is having computer issues I recommend the following.
1.Open "My computer", go to your "C" drive and right click on properties. Click on tools and schedule an error check after checking both boxes to automatically fix errors and scan for and attempt recovery of bad sectors. Restart your computer and let scan commence.
2.Go to www.cnet.com
and download Malwarebytes. Install program, check for updates and run full scan. Manually check for updates about once a month and manually run a full scan weekly.
3.Go to www.cnet.com
and download "Super anti-spyware free edition". Install program, check for updates and run full scan. Manually check for updates about once a month and manually run a full scan weekly.
4.Go to www.free-av.com
and download Avira AntiVir as your free anti virus program. Install, check for updates and run full scan. This program will monitor your computer as you surf and if set up correctly will automatically download updates. Run scan at least every week.
5.Go to www.cnet.com
and download "CCleaner". Install program, leave on default settings with one exception. Go into the settings area and check one of the boxes for secure file deletion. I use and recommend 3 overwrites. This program should be run weekly and will remove a tremendous amount of garbage from your typical "abused" home computer.