Windows Defender Offline (Offline Virus Scanner)

Everyone should have an up-to-date real-time anti-malware application running on their system at all times.  These programs won't guarantee that you will never get infected by a rouge application (e.g. virus, Trojan-horse, worm, root-kit, etc.), but it provides better protection then not having this software running at all.

If your computer starts acting strangely, you might want a second