This question and many similar like it, I answered on Y! Q & A. In this case the system could have been compromised. See my answer below.
Since your system may already be compromised, I would first advise that you get an online scan done. Make sure and back up your files off of your system first. (you could always put them back and scan them after)
The best one I know of is from bitdefender (Link 1) Just click start scan but remember to check that you have the correct system requirements i.e. your operating system is in the list.
Once this is done go to the second link( Link 2), there I already have listed some of the best security tools I’ve ever come across (Look for Safety and Security). The links will take you to the original creators/makers website. You can choose between Avast or AVG it depends on you.
When you reach to the respective sites look for the version that would accommodate your system.
Download, open, install and update and run a scan using each of the software (except the firewall of course. that you just install and configure). ONE at a time. Multiple security software that run at the same time causes issues.
Once you have scanned you system and clear it of any viruses/ malware that you may have.
You need to keep the specific security software updated and run scans regularly.
Run defragments and Disk Cleanups (check in your system tools) every now and again. And it’s wise to also perform a system restore every now and again as well.
Use an internet browser such as Firefox or Google Chrome or Safari
Oh and remember to scan every file that comes onto your system and every media (USB, CD)
If there is something that you really don’t want the scan to mess with on these media . Zip them.
Sorry this is so long. I hope this helps. Feel free to PM me.