I built my own Windows Home Server box.  You can buy some decent pre-built units, but I wanted to build my own.  I wanted a small, quiet, low-power device.  I was willing to compromise on performance & capacity to get it.  I decided that my collection of movies should live on my media center, just because I wanted to have a small WHS.

Dell D600 laptop

A laptop meets the small & low-power requirements without effort.  They also have a built-in keyboard/mouse/display and battery backup, while still being small.

Dell leases these to corporations.  After the lease expires, they are sold for cheap.  I bought mine at a popular auction site for ~$300.  It had a 1.4GHz P4 and 512MB RAM.  Most have USB 2.0 (important for adding storage) and GigE (important for pushing a lot of data back and forth).  Many have Wi-Fi, which may be useful.

D600 parts are easy to come by, and their online manuals are very good.

Pair of 160GB 2.5" drives

Hard drive prices (per GB) follow a "saddle" curve.  At the time of building, 120GB were at the bottom of the curve; 160GB were a bit higher.  Above 160GB were much higher.  I knew that I wanted space for 100GB today; 160GB would give me a little breathing room.  Since replacing drives is expensive (and replacing the primary drive is annoying), breathing room seemed wise.

For the secondary drive, I bought a 2nd HD caddy, which replaces the DVD drive.

Great print server!

It's small enough that it can fit comforably under my printer on my desk.  (I was careful to leave a little venting room.)  I like simple, obvious names, so my printer path is now \\SERVER\Printer.  The USB cable from server to printer only spans ~6 inches.

What if I need more storage?

 One of these days I will probably decide that my movies should be moved to the server.  I rip my DVDs lossless, and currently have ~400GB, so I'm going to need more room.  Upgrading the internal 2.5" drives won't be feasible for a while - they're still too small.  Until then, I will need to add external storage. 

I can use the pair of USB 2.0 ports.  With these, an external enclosure with a pair of large 3.5" drives are a good choice.  (Two drives means I can fully use the USB bandwidth without dramatically overloading it.). 

Another option is to get a PCMCIA card (there's only one slot) with USB 2.0 or eSATA. 

One downside is that the external drives won't be on the battery that's built in.  That means I will need a separate UPS if I want them protected.