There are many different types of viruses out there that can infect your computer if you don't have the right protection or take the right precautions when web surfing or opening emails. Most people have some sort of antivirus software installed on their computer but many times just having that one program is not enough because it's hard for one program to be able to defend against all viruses.
If you think you have a virus and your antivirus software is not finding anything when you run a scan then you may have to try out some additional virus removal tools.
Kaspersky Labs offers free downloadable virus tools for a wide variety of modern viruses. All you need to do is go to the site and find the virus you have and see if there is a tool to fix it. http://www.kaspersky.com/removaltools
They also offer free trial versions of their antivirus and firewall software for home and business use. Plus they provide up to date information on current virus and spyware threats.