There is no reason to EVER pay for antivirus software.
For one, many malware developers will test against Norton and McAfee before releasing their viruses into the wild.
This antivirus doesn't have you restart your computer upon installation, doesn't require you to register, updates and scans automatically, and is completely free as long as you have a valid copy of Windows. It's great for detecting rootkit activity, which is pretty rare.
Piece of advice: if you want to be extra-safe, couple the protection from this with an occasional up-to-date Malwarebytes scan (malwarebytes.org).