When I go to close my website project, I get a pop up box telling me "the requested resource is in use" and the program will not close.
I don't have any idea what the resource is or how to find out what it is????

(Version 12.7, build 454 on Win7 64bit)

Does it do it even if you force closer then reopen?

I'd upgrade to the latest build (456) then see what happens

I haven't had time to re-open and give it a try (just checking in before I go to work).
I tried using the split-screen view, but kept getting constant pop-ups about active-x controls, so switched off the split-screen and worked another hour or so, saved and clicked the close all option. That's when the message came up. So I'm thinking it has something to do with the split-screen/active-x....???
I did a re-start on the computer and opted to force close the Editor.

Gunsmoke, I'll get the latest build later today

It may not be the problem you have but there is a program called unlocker. You know those times when you want to delete or close a program and you get a message that it is used somewhere else, well that makes Windows withdraw its hooks on program or file.
I get the split screen to hang sometimes when I inadvertently try to view php code, so it could be an unresolved script causing it to hang. Consider to use localhost in Wamp or equiv. in Xamp for previews when you dump in the clever stuff.