It doesn't have to be necessarily a single process
If I sum up all processes, memory consumption is around 1 - 1.5 gigs. But it is the μTorrent client to blame. Even though the specific process associated with the client, reports a few hundred KBs of RAM consumption, it actually must be consuming much more. When I exit μTorrent, memory is freed almost instantly. Probably a memory leak. Vista is up to date btw. I also found that running the torrent client as admin helps a little bit - memory usage builds up at a slower rate. When it reaches ~80% I quit the client and restart it.