Thanks for responding, Pyro.
Just tried it and Alt-F4 works like a charm. Then I double-checked selecting "Quit" from the initial clipboard and it still hangs.
Alt-F4 seems like an acceptable workaround tho. Any other Windows 7 users having problems?
After I submit this last batch of P-38L skins I've been working on, I intend to back everything up and try uninstalling anf then re-installing.