game file extension...(???)

Two games I recently downloadede had the file extension ".nes" as opposed to the usual ".smc" I could get neither to run on snes9x or zsnes. Any tips? Is there a way to find out beforehand which it's gonna be? Any way to get them to run? They start, I get the black screen and that info in white at the bottom of the screen, then nothing. Just darkness. "What doesn't kill you doesn't make you stronger, it just doesn't kill you."