There are a lot of antivirus software in the market nowadays. Some are free but some you need to buy it and it expires according to how long you subscibe to its services.
I had been using and tried a few antivirus nowadays but none are reliable and get the job done like Avira. Its lighter than Avast and Norton, so it does not slow down your PC.
Most important, it comes for free and the protection you get from a free antivirus like that well I think it is worth it. Personally I have not try the premium version but of coz it should be better.