Possibly CryptoCoin Mining Software
I have read that some untrusted game crack uploads to torrent sites can also install cryptocoin mining software (i.e. Bitcoin, Litecoin, etc...). Basically the torrent uploader adds cryptocoin mining software to the installation files for the cracked game, and directs the proceeds of your GPU's work to their wallet. Its actually pretty smart, these guys know that if you are downloading the game that you have at least semi-decent hardware, and they take advantage of that. I also see that your post dates during the time that the pirate bay was down, in which a lot of shady look alike sites popped up. I am not saying, nor am I condoning, that you pirate copyrighted software. I'm just saying its something you might want to consider.
As far as I know, antivirus does not pick up on the cryptocoin mining software because it is legitimate software. If it turns out that you do, or believe that you may have cryptocoin mining software on your PC, I would suggest a fresh OS install. You could probably clean it, but there could be remnants of the software remaining, and they could have taken measures to make it hard to find the directory. Just my thought on the matter. Also, rolling back drivers would definitely not fix this issue, and it is likely that a restore point would not help either. That's why I suggested a clean OS install. I realize this post is a little late, but perhaps it could help someone else.
I would like to reiterate for the mods that I do not condone, nor do I myself, pirate copyrighted material. I was just relaying some information that I have run across in my interwebs exploits for the OP.
Edit:
I forgot what led me to this conclusion. The fact that your card is running hard, but not overheating, makes me think that it may be a cryptocoin miner. Your card is running at 90%: (1) indication is that there is either a graphically intensive program running, or (2) a program that relies on heavily paralleled computations best suited to graphics cards. Examples of #(2), and the only ones I can think of off the top of my head are video rendering software (like handbrake) and cryptocoin mining software. Also, your card is running hard, but not running hot (considering the load it is under) indicates that it is not a cooling issue. If your PC would not boot, then it could possibly be a power supply issue. When your card isn't being supplied proper current and/or voltage, AMD cards (not sure about NVidia) will kick the fans up to 100%, but the system will not boot. I guess it could be a an intermittent problem with the power supply, but I think you would get random crashes or blue/black screens rather than just an over-active card.
Edit 2:
Oh, and also, if the source of the problem seems to be steam, then it is possible that it is indeed a cryptocoin mining program. The game cracks have to include a steam.dll file to bypass DRM measures in place. Seems a logical place to put some additional software that you don't want the installer to know about. Especially since the steam.dll files are flagged as a virus (for any cracked game) by most if not all antivirus programs, and the installer must turn the antivirus off or bypass the antivirus security measures in order to install the steam.dll file.
Oh yeah, and I really don't condone piracy. But, I do download freeware and such through torrents, which is why I educated myself on the risks of downloading from torrent sites. :devil: