How long have you been on Internet? And you don't have any Antivirus software? That's certainly a big risk. Antivirus software utilities are expensive? Yes, it is true. Many of these programs are expensive, but they are absolutely essential.

Once installed on your system, and if kept properly updated of all virus definitions, an antivirus software utility will scan all files and separate suspicious from safe. It will alert you if a file is infected and either delete it or quarantine it so it cannot cause havoc with your machine. That way you system is constantly protected from destructive behaviour of viruses. You will not be at risk of infection and, since a virus replicates itself, you will not risk infecting your friends. Still, if your budget is too limited to include some of heavyweight antivirus solutions such as Symantec's Norton, McAfee or Kaspersky, there are some free options available if you know where to look.