Everyone's suggesting stupid stuff to you. I worked a for a long time with virus and products related to moving them. I'm even a Malwarebytes Affiliate.
Get Microsoft Security Essentials and Malwarebytes. Both are free.
Their is a paid version of Malwarebytes which offers some on demand protection, which I highly recommend if you can spare the cash. Malwarebytes pro is only $25 and is only a one time Payment.
The reason I recommend both is because Security Essentials will keep out MOST things but Malwarebytes is probably one of the best scanning software's available, and will clear out anything it misses.
They will not conflict with each other and work great along side each other. I don't use Essentials and still Love AVAST however I'm told essentials is great and from what I've scene of it on PC's seems to be fine.
Security Essentials:
http://windows.microsoft.com/en-GB/windows/products/security-essentialsMalwarebytes:
http://www.malwarebytes.org/Malwarebytes PRO:
https://store.malwarebytes.org/342/?affiliate=2169&scope=cart&cart=29945If you can afford pro PLEASE use my link as like I said I am an affiliate.
Anyways hope this helps. Feel free to ask questions. Also I'm usually always available for virus removal assistance. I worked doing that for a while tbh. And my Assistance is free XD.