Recently, we released the only official Windows-specific version of ClamAV, appropriately called ClamAV for Windows (http://www.clamav.net/lang/en/about/win32/). It is designed to use little memory and processing speed because it uses an advanced cloud-based protection mechanism, best of all it's free (as in free beer. Ummm...beeeeer). If you haven't tried it yet, I really encourage you to.
Ransomware in action on a PC
Ransomware being detected and it's actions blocked by ClamAV for Windows