welcome to the forum. it sounds like you have been hit by a malware that is not removde. i have have simler problems a few mount ago. i run a scan with spywareterminator and it find the bad guy a trojan, vundo sample. a side note there. to your problem: try a a scan with malwarebytes antimalware and see what it comes up with.

http://www.filehippo.com/download_malwarebytes_anti_malware/

download install update and do a scan, don’t forget to remove what it finds a system reboot might be needed also.

second you could also try a boot scan with avast and see it avast could get the bad one

http://www.schmahl.net/avastbootscan.php

i also add spywareterminator as a second opition for if avast and malwarebytes antimaware should come back clean.

http://www.filehippo.com/download_spyware_terminator/

good luck and let us know on the progress or if you need more help.