Online virus scanning
One of the first things many modern viruses do is circumvent the protection offered by popular anti-virus programs such as Norton or Mcaffee. It is a good idea to run one of the folowing free virus scanners periodically to clean up anything that may have wormed it's way in.
You can scan your computer for free using Trend Micro's Housecall
One of the best things you can do to avoid viruses is frequently update your computer using windows update.