Is your computer displaying odd behaviour? Have you lost data? Suffering from problems starting up? You could be infected with a virus. Usual routes of infection are peer-to-peer downloads and email attachments.
Is your computer running slowly? Seeing dozens of infuriating pop-up windows? You may be infected with spyware, commonly a result of browsing malicious websites while using Internet Explorer for Windows. You can also become infected by installing peer-to-peer software from certain ethically questionable companies.
As the old saying goes, an ounce of prevention is worth a pound of cure. If you are using a Windows-based PC, you must perform regular spyware sweeps and anti-virus scans with up-to-date programs. I cannot emphasise that second part enough - if your virus and spyware definitions are not current you might as well not bother. If your PC doesn't use the Windows operating system you are far less susceptible to infections, but you should still perform a scan every few months.
If all else has failed and you think your computer is infected beyond your ability to repair, give me a call and I'll see what I can do. In most cases I should be able to solve the problem in an hour or two.