You may want to try these at home on your school laptop or desktop. They are all free. They also offer versions that you can purchase. Copy and paste the web address into your browser address line.
Avast - http://avast.com/eng/avast_4_home.html (for pc)
Click on download on the left. You must meet the conditions of home users and non-commercial use to get the free edition. This one will set up and run automatically in the background. There is also a mac edition.
http://www.apple.com/downloads/macosx/networking_security/avastantivirusmacedition.html (for mac)
Spybot - http://www.safer-networking.org/en/index.html
Be careful with this one. There are other programs out with slightly different names. These actually may put spyware on your computer and then you purchase their program to clean your machine. You must run this one; it is not automatic.
CCleaner - http://www.ccleaner.com/download
Scroll down and click on "Download from FileHippo.com (in the same box as the paypal donate). On the right, click on download latest version. If you take all of the defaults, you will install yahoo toolbar, so you may want to uncheck that box when it appears. You must run this one; it is not automatic.