Here are some free resources to keep your computer safe. All these downloads are field tested and I use these in at home, in teaching, and in consulting.
The Enhanced Mitigation Experience Toolkit (EMET) - A free tool from Microsoft that stops threats your antivirus doesn't detect.
Microsoft Security Essentials - Free antivirus for computers running Windows 7 and Windows Vista
Microsoft Windows Defender - Free antivirus for computers running Windows 8.1.