Boot into Safe Mode with Networking by tapping F8 repeatedly while booting up and re-run Malwarebytes (Free is best), update Malwarebytes and run another scan in that mode.
If it still finds nothing then try winding back time by initiating System Restore to before all this happened.
You'll find that either in Start/All Programs/Accessories/System Tools or go to Start/Run and type in rstrui.exe and click OK or hit the Enter key.
Any idea what may have been downloaded in your absence?
I really have no idea. I mean I consider myself quite careful and very computer savy, the first thing I did was system restore but my only restore was on the 15th. Still has the problem.
Did you try instead of Safe Mode in that list of options, "Last Known Good Configuration"?
That sometimes works surprisingly.
The other thing I can suggest you do is post a Hijackthis log on one of the forums that specialize in them and the details of that are listed at the bottom of this document I've thrown together:
The only other thought I have right now as it was left on, is that perhaps a power surge has damaged something and of course of that's the case best to seek a PC expert's help.