Xbox 360 = console for the sole purpose of gaming, watching videos, and playing music.
PC = a computer. Of course PC's have a calculator, flash, web browsers, that's because they're computers.
They both can be used for gaming, but cost-wise you have to shell out a lot more dough for a quality gaming computer that will last you a few years, rather than dishing out 300 more or less.
The xbox is one of the most modded consoles in the world. Have you been on hacknmod.com? Of course you can't customize the hardware as much, but that's because you don't need to. It already plays all released games the way they were meant to be played.