I'm using version 2.5 on Windows 7. On 2.10 I had this same problem, and disabling Aero fixed it.
ZQuest works perfectly fine, but ZC just does not want to start at all. Every time I try to start it the screen flickers for a second and nothing happens. I've tried running it as administrator, in compatibility mode, disabled Aero, disabled visual themes, ran it from the launcher. Nothing seems to work at all.
Is there any other way to fix this?