Hey there,
Well I know a program that is 100% free and it isn't just a trial, so you'll have it forever =)
Just chose the site in which hosts spybot and download it
Once it has fully installed open the program, you'll get a message just click OK and you'll get to the main page.
From here you can either
Check for problems
Recovery
Search for updates
Click on the button that says Check for problems
Then spybot will automatically start a scan, there is a progress bar at the bottom of the screen, it should take a few moments, but it's normally rather fast compared to other scanning programs...own oppinion though =P
After it has finished scanning it should have a list of problems found (if any)
Just click the button fix selected problems at the top of the page.
After a few moments the problems should have been fixed.
TA DA! And you're done!
To make sure you're computer is "Immune" to known viruses on the right hand side of the screen there should be icons with names under them.
Click on Immunize then a message should come up saying how many known viruses are blocked on your computer.
Just click OK and spybot will block these viruses from your computer =)
You may need to go back ever few weeks however as more and more viruses are discovered.
I suggest running a scan once ever few days =)
Hope I was able to help.
Keep us updated on your progress