I use malwarebytes for spot cleans. The free version works great, you just manually have to start the scan - http://www.malwarebytes.org/products/.
Is you're PC and web browser update to date?
Do you use an anti-virus?
Are there any other symptoms with your PC like it's slowing down, freezes, new prompts you haven't seen before, etc?

I would be cautious of doing anything personal on this PC until you figure out what's going on (ie banking, facebook, etc)