"Cheapest" is "free" and Avast is by far the best free software.
Windows Defender (basically a renamed Microsoft Security Essentials) isn't much good...and even Microsoft admits as much.
If you want to pay, then Kaspersky is now widely regarded as the one to use.
Another good option is Malwarebytes. The free version offers pretty much everything the paid one does except for live protection, but it updates its database every day.
Avast and AVG are very good - never tried AVG. Had Avast for a long time but it gradually seemed to slow the system down, but it may just have been my rickety old machine.
Use a combination of Avast and Malwarebytes is good. Set flash to 'ask to activate' if you really need to use it (a lot of websites are using html5 now). Don't use java unless absolutely necessary. Surf net with a restricted user account. Use a good adblocker. Disable on trusted sites that require revenue from adverts such as this one.