I am running Windows 7 x64. Some time ago I was trying to modify Windows themes using something I downloaded from deviantart. I remember some of the files installing a bunch of unwanted trash and needing to modify system files, although since it was a long time ago I do not remember which ones. After installing something from there, Windows Explorer has stopped executing 64 bit applications: double clicking on the .exe file simply does nothing, right clicking and selecting "Open" or "Run as Administrator" is also ignored. 32 bit applications execute normally, and launching 64 bit applications via other means, such as a third party file manager, also works fine.
I have tried locating the problem, but haven't found anything out of the ordinary with system files. It is also not a problem with uxtheme.dll, since I specifically checked that file. Reinstalling Windows is not an option. So the question is: what are the libraries that Windows Explorer uses to execute files and which of them could have become corrupted without issues spreading beyond the scope of this question?