Is this Windows 7 SP1 and fully updated including IE11 (whether or not you use IE is irrelevant)?
If it keeps coming back then either you have something attached, a hard or flash drive, or network, that is infected or at least insecure and letting someone in. Probably the best solution would be to completely format and reinstall the system but I doubt that you have a system disk, am I right?
if you can boot into Safe Mode with Networking try Malwarebytes Free as per the last link in my signature below.
Also in the last link in my signature below, lower down...I suggest you post a Hijackthis log on one of the specialist forums I listed there for some expert free advice.