Using old hardware to make a torrent box.

I build a small wiring closet in an empty closet in my house with my modem and router. I plan to also put a small machine to download torrents for me to.

The specs will most likely be crap I have in my basement:
Pentium III ~450Mhz and matching ASUS motherboard. Probably very little RAM, assume a single stick of 128mb. Generic video card, not even sure if it will be AGP. I'm probably going to have to buy a small harddrive around 20-40Gb. The machine won't have much cooling since it'll need to be quiet.

The features I'd like are webserver and torrent client+web interface. I want the most lightweight OS and software.

What are you suggestings?

I had a little old box with 128MB ram, 450mhz cpu, and a 4GB and a 40GB hd in it. I ran torrents on to it all the time and it never had any issues. I was running Windows 2003 server enterprise ed. and it ran absolutely fine. the most useful bit was that I had complete control from my laptop using the Remote Desktop Connection. looking at the setup you're considering, you should be able to run 2003 just fine.