Download Malwarebyte's Anti-Malware, Ad-aware and Spybot, and run them all (one after another, not all at the same time) to see if you're not infected by some spyware.

Also run some anti-virus, like Avira - or even the online Trendmicro housecall (http://housecall.trendmicro.com/housecall/) to check for viruses.

And do that if possible disconnected from the internet.

If nothing is picked up by those programs, then you can try a system restore to a point where everything worked. Be sure to backup your data first, since you can loose some data with System Restore.