I want to run xBox 360 games on my computer (I can't stand the controller, and love my mouse and keyboard). So is there an xBox 360 emulator available for Windows 7? If not, what is required for there to be one, eg. why isn't there one yet?
No, there are no Xbox 360 emulators. The most basic reason is that the Xbox 360 uses a triple-core 3.2 GHz PowerPC processor, and desktop PCs use x86 processors.
Because they are different CPU architectures, the emulator would have to translate the PowerPC machine code into x86 machine code on the fly, which takes a significant amount of processing power. As a general rule of thumb, expect the emulated software to run 10 times slower in an emulated environment. So we would need triple-core x86 processors running at 30 GHz.
(There are many other factors, and this is just the most basic first hurdle. These numbers are also just ballpark estimates: there is a lot of variation in emulation methods and different processors are not directly comparable on a clock speed basis. But this gives you the basic gist of the problem.)