If you are talking about antivirus for Windows I think you should rather think about Windows defender or Microsoft security essentials which are free and should be sufficient if you add a bit of caution when browsing the internet or reading email.

Open source alternatives like clamwin require more care as they don't monitor access in real time unless you install something like clam sentinel and that's a bit more time consuming and more difficult than using the free alternatives from microsoft itself.

Whether it's worth buying Norton or McAfee is a matter of taste, but for me it's not worth it.