Windows Defender in 8.1 is absolutley superb. Way better than nearly all virus killers available to buy. Don't believe all the crap you read from the IT media.
In conjunction with the built in Windows Firewall, I have yet to get any sort of virus, bot, worm or rootkit using the built in 'tools'.
Yes... I have been to more than a few 'dodgy' websites. Defender always catches any nasties.
Also bear in mind that on Windows 8 onwards, Defender is basically the excellent MS Security Suite, which used to be a stand alone program.
Currently using the bulit in virus and firewall in Windows Tech Preview, and they have both performed admirably.
I would not recommend using AVG either. Yeah, it's free and one of the better free ones, but is does impact system performance quite a bit.
A virus killer should be silent and very light on resources, which Defender certainly is...
I've tried nearly all the top virus toolkits and firewalls. Most, if not all, are a severe drain on system resources, causing web slowdowns when none of them should.
After 30 years of developing for PC and phones, I would heartily recommend Defender and Windows Firewall as more than capable of protecting your computer and precious files.
IT media always quote skewed results for virus tests, and in my opinion, is not a true test of a virus killer's effectiveness.....