Any site which offers a free online virus scan is almost certainly trying to sell you something. AVOID!!!
To amplify on CuteToni's post, the essential security elements on your PC should be a firewall, an anti-virus program and anti-malware sofware.
You should have just ONE firewall running (to avoid software conflicts). From XP onwards, Windows has included its own firewall which runs by default. It's less than perfect; most people would suggest that you should use the free version of either Zone Alarm or Comodo.
You you should also have just ONE antivirus program running. The most popular free products are AVG and Avira. (Those programs scan incoming files and also carry out scheduled scans for anything nasty).
You can have as many anti-malware programs as you like (because they run 'on demand', rather than 'in the background', so avoiding system conflicts). The most popular products are the free versions of AdAware and SpyBot Search & Destroy, together with the excellent MalwareBytes.
Commercial software (such as that offered by Norton or McAfee) usually provides a complete suite of programs, to cover all three functions. However such software can often slow your computer and provide little (if anything) which you can't get for free.
Chris