This is on of those projects, which can be titled, do-it-yourself. Many people perceive Home Theater Personal Computers (HTPC) as nothing but a fancy file changer. Here's what Tomhardware people did. They decided to build their own low-power home theater PC from scratch in order to inspire others to do it themselves.

It was not all from scratch but they used many unused and spare items lying in their office. However, the product of their hard work looks good.

They started with the selection of processor and zeroed in on 35W Athlon643200+ energy efficient small form factor CPU. Then for the Motherboard, they found Asus M2NPV-VM.
The graphic card used by them was GeForce 6150 graphics, which consumes so little power that the Northbridge can be passively and silently cooled, even at a high enough performance level for viewing MPEG2 and MPEG4 video at common 720p and 1080i resolutions.
For the case, they picked up Zalman's HD135, which fitted their bill, with an appearance similar to 17" by 5" home theater receivers.

They chose the PowerColor Theater 550 Pro PCI-Express x1 card TV tuner card. Theater 550 based cards have become the TV quality reference product for many sites, though ATI has recently added features in its ATSC-capable TV-Wonder 650. PCI-Express tuners are also available from other ATI partners.
Other components were chosen from here and there or were borrowed from other PCs.
Then the assembly and voila!! The CPU is ready for action until next season till they will eventually will have to upgrade it with the changing technology.
For more details check out Tomhardware.