To meet your requirements, go for:
EPIA 5000 - silent due to passive CPU - even running Win2K, has adequate performance, even under Win2K.
If you need more performance, go fan the fan cooled EPIA800. (I use an EPIA800 in my Win2K file server, with a Noiseblocker northbridge cooler fitted).
2688 case - silent as it has a DC PSU, leave the 2 case fans disconnected
Seagate Barracuda, Samsung Spinpoint or Maxtor - all very quiet and as fast as you need
slimiline optical drive - choose whatever your budget can stand, or features you need. ie. cd-rom drive, dvd rom, cd-r/w drive etc.
256Mb PC133 - Use decent RAM, we use Kingston, it works without issues. 256MB is more than adequate for file serving.
This combo is as cheap as you can get, but not the smallest. The smallest silent option would be the LEX Light...
You can find all these items in our store.
Hope this helps