spyware is different to a virus , spyware is unwanted programmes which install on to the host machine (that's your computer) without the users knowledge and/or consent , these programmes then go about tracking and watching everything you do on the Internet and build a profile of what you like in the event to show you pop up ads to things you are interested in, this information is then shared with any interested parties.
These are a pain install alot of the time without your knowledge and can hog system resources and slow your computer down , sometimes to a halt but there are ways of fighting back and a few rules to follow when surfing and downloading on the Internet
1. beware of trick advertisements which make it look like it is part of your computer , never click on the directly to close it (even if it says it will) just click the cross in the top right part of the window
An example of a clever ad pretending to be a spyware scanner , but by clicking on it you invite the hidden spyware tracker to install on your computer.
2. When installing software which you downloaded off the internet make sure its from a source you trust , sometimes a useful application can be hidden with spyware such as games, spyware scanners etc...
3. Install adaware and run scans regularly , i recommend maybe 3 times a week, you can start the scan when u wake up of a morning and let it run while you're getting ready , run a scan while your going to lunch or to dinner or even leave it on over night and schedule a scan when no-one would be using it.