Thief cannot start when Steam is not running?

[Lorto]

I was able to launch the game once. Now when I try and launch it (from within my Steam Library, no less) I am greeted by the aformentioned text.
Steam is running, I am not offline, I have validated all local content and now I am perplexed.
Any suggestions?

Windows 8.1 (64-bit), i7-4770K, GTX 780 Ti, 24 GB RAM

[avils.shadow]

any solution?

[Aksu]

All I can say is that everyone of those who finds this problem ( me included ) are using windows 8.1 :(

[DX]

i got it working by simply exiting steam.
go to steam.exe and set colpatibility to windows 7.

[Hichem]

never buy game again, go sea