If you are not careful about what you install on your computer, not even an anti-virus can help you out.
I kept my old computer for 4 years without re-installing the operating system, and it was windows XP. What ? You can keep windows XP for 4 years without re-install ? Yes, i can. I clean-up every month, run virus check, defragment, delete old registry entries, remove unused programs, keep directories happy and i kept the same windows installation from 2004 to 2008.
What happened then ? Well, i had kaspersky anti-virus installed, and i wanted to play online poker. I played before that many times, with different poker software, but they keep updating them every week so i had to download a new version.
My anti-virus did not recognized the poker software installer as a virus, but he warned me that it has suspicious behaviour. Well, many programs are considered with suspicious behaviour as i've set the anti-virus to tell me everything, it tells me every time even if it is a trusted software.
The poker software did not contained a virus, but he started to show me offers to buy things in the sidebar, pop-ups opened, and suddenly the windows started to work very very hard as it was a computer 4 times weaker than it was before.
I tried ti clean it up but because the anti-virus worked very hard to block the pop-ups and messages that appear, and to stop the software to enter in more areas of the computer made be unable to do the normal clean-up process so i decided it is time to try something new, and i installed Linux.
I had that windows 4 years and maybe it could have been more, but it was my mistake because i installed software that i should not trust. So, be careful at what you are installing: better safe than sorry.