Virgin/sky costs money. You can't add a blu-ray drive to those boxes. The old virgin/sky boxes don't have music, netflix, or any other type off apps/content.
A hTPc allows you to do pretty much anything you want.
You don't have them over a normal pc, they are two different things. But can sometimes be one and the same. But they have two very different purposes and are usually two very different builds.
As a hTPc isn't about power, its about small and quite. Which means using lower power parts to keep heat down, so you don't need noisy fans.