HTPC-How are they built, and what do they do?

[Abarine]

I'm planning on building an entirely new rig for myself, and will have some pieces left over from my old one when it's all said and done. I was considering building an HTPC with the pieces that were left, but then I realized that I didn't really know anything about that one way or the other.

So, I guess my question is what is are the uses of a HTPC, and how does one go about building one? I know it sounds rudimentary, but I really don't know.

[irwincur]

The same way as any other computer. However, the goal is typically to keep it quite and relatively powerful. Other than that, you really need to know what you want out of the end product.

[ImaNihilist]

Some things to keep in mind:
Windows XP Media Center Edition
Remote or wirless keyboard and mouse

[Adisharr]

If your going to use it for media mainly (no games) you don't need anything powerful. I'm running Showshifter on a P3 700 / 256MB ram and saving / loading the recordings to my main PC in the basement. Anything over a 1 gig P4 will handle just about anything. There are a few software packages that will handle 2+ tuners as well. Showshifter only handles one now but they are working one a multi-tuner version.