I've never come across a genuine emulator, they're all fakes that infest your computer with malware. Even if you do find one, don't expect it to perform as well as it would on a real Xbox 360, even with a high-end PC.
The other problem is that emulators don't use games loaded from disc. They require an .iso file which is the contents of a disc contained within a single file that PC programs recognise. It may be possible to download them, but it's illegal to do so.
If you want to play those games, buy yourself an Xbox 360. They'll be going cheap these days.