Viruses and Malware
The most important thing you can do, is install an anti-virus product.
There are a number of free options, a number of paid for options, and a number of rogue (to be avoided) options, so please choose carefully. (see our recommendations at the bottom)
First and foremost, never install and Anti-Virus product that pops up a window from a website telling you that you may be infected. This is a virus itself, and must NOT be installed. (check out our rogue AV page)
Originally, a virus would be transferred from one computer to another by means of a floppy disc. An infected computer would install a virus on to a floppy that was installed, which would then hop into the next computer it was inserted into.
Those days are pretty much long gone, with the advent of the internet, so that now the most popular way of getting a virus is via email.
Another popular way of getting a virus is from the web, by visiting a malicious website, which installs a virus in the background without you knowing.
You can also get a virus by downloading illegal movies, music or software. Peer to Peer networks and Bit Torrent sites are riddled with viruses and malware, which are files named to appear to be songs, movies or computer programs that users download thinking they're getting a movie or a song or a program.
Just don't ever use these sites. For one, you're downloading illegal copyright material, and for two you'll run a very high risk of getting a virus.
Avira AntiVir: Also free
Anti-spyware and malware: