I have tried to run Construct 2 through Steam with multiple XPs and it doesn't work. Steam just says "Failed to start game (app already running).
Today I just got Windows 8 installed on my Mac, which I bought only to use C2. Does it work? Nope! The exact same error message occurs, and I really doubt it has anything to do with my graphics driver which is fully updated.
This is a major bummer, as I can't switch to Windows 7 now. The only OS C2 has worked with is W7 in my experience, which I borrow from a friend when it's available. The reason I bought W8 was to test out my games as W8 apps.
I think that you should consider fixing this bug as it impacts a LOT of users. I posted this problem on the Steam page as well, and others have answered that they have the same problem and don't know of any solutions.