Modern uTorrent leaves several background services running even after you close the window. Version 2.2.1 Portable dies when you close it. It doesn't phone home, it doesn't update, and it doesn't harvest data.
The executable is remarkably small (often less than 1 MB) and consumes very little RAM even when seeding hundreds of torrents. utorrent 2.2.1 portable
This build is 32-bit only. On 64-bit Windows, it runs under WOW64 (Windows-on-Windows 64) with no issues. it doesn't update