This is a never-ending issue on computers, and one that really comes down to paying attention to where you go and reading, yes reading, pop-up windows if/when they occur. There is NO single solution when it comes to virus/malware/spyware, but with the right combination and forced vigilance, you can keep your PC clean.
I use, and have been using, AVG-Free for the last 3 years and put it on my parent's computers last year. I have not had an issue with viruses or malware in that time. AVG has the smallest footprint with regards to memory/cpu over the average. I also run Windows Defender with decent success. It picks up some of the stuff that gets through AVG.
IF something gets on your PC or you didnt read or just clicked through something, Ad-aware, SuperAntiSpyware and MalwareBytes will get 99% of what comes down. With those, you do NOT have to keep them installed and running. You can install, update, clean and then uninstall them.
The best malware and spyware and virus protection you have is just reading. Keep in mind, NO internet website can do full scans of your PC. If one says it did - run away. Hit ALT-F4 to close the window - do not click anywhere in the window.