uTorrent:

uTorrent is an efficient and feature rich BitTorrent client for Windows sporting a very small footprint. It was designed to use as little cpu, memory and space as possible while offering all the functionality expected from advanced clients. With BitTorrent, you can download files faster and contribute by sharing files and bandwidth. Features: Written from scratch in C++ with efficiency in mind; Manages multiple simultaneous downloads; Smart bandwidth usage; File level priorities; Global and per-torrent upload/download rate limiting; Quickly resumes interrupted transfers; Universal Plug&Play (WinXP only); Supports popular protocol extensions; Low memory footprint and decent speeds; Attractive Interface: When hunting for mini-BTclients, interface it's usually the first thing that goes out the window for the sake of filesize / processing. This is not the case with uTorrent. All information that I routinely check is represented (file listing, pieces, tracker details). This BitTorrent client seems to have all the best features of programs like Azureus or ABC but is under 100K of size ! No Install Required.