Figured it out after an exhaustive period of trial and error which made me grateful for the 30 second restarts my SSD's afford me

It's AtiTrayTools. As soon as I stop that auto starting, everything is fine.
Edit: after more investigating, it's actually a DirectX issue. Seems the bundled copy with Win7 doesn't have everything it should, once the redist is installed (i.e. when trying to install a game) it puts the extra bits in. I installed the copy from my Magica Steam folder and now everything can autorun

. Weird issue since steam would work provided ATT wasn't started, but I guess it must force a particular mode or lock some files or something.
I'm hoping this will fix the mumble overlay issue i had too
