Norton Anti Virus will take care of viruses as long as you are getting the automatic updates.
As MrMoo suggested,
download Adaware and install it. It's one of the best free utilities for detecting and removing spyware, dialers, trojans, etc.
If you browse the Internet on your PC, you may also want to
download the Firefox web browser and use it insted of Microsoft's Internet Explorer web browser. It's free, and a lot more secure and less prone to hacks and hijacks.