If you see new toolbars in your Internet Explorer that you haven't intentionally installed, if your browser crashes inexplicably, or if your home page has been "hijacked" (or changed without your knowledge), your computer is most probably infected with spyware.
Even if you don't see the symptoms, your computer may be infected, because more and more spyware is emerging. Spybot Search & Destroy is free, so there's no harm giving it a try to see if something has invaded your computer.
Spybot Search & Destroy can also clean usage tracks, an interesting function if you share your computer with other users and don't want them to see what you have been working on. And for professional users, Spybot Search & Destroy allows you to fix some registry inconsistencies and extended reports.