There are many threats on-line, the most popular being viruses and spyware. If you are infected your PC's performance will be noticeably slower and there may be other signs such as pop ups, settings being changed and your PC restarting itself. The majority of infections are just a nuisance and can be removed without much trouble, but some can be a danger to your personal information, data and PC's health.
Time is important!
If you think you are infected it is important to get it removed as soon as possible to prevent the infection spreading.