Urban Terror 4.1 can run on Quake 3 Arena or on the free ioUrbanTerror. The .zip and the .exe installer have exactly the same files in them and both include ioUrbanTerror, so you only need to get 1 installer. There's only FULL packages, no upgrades. The installers are about 700MB.
The .exe installer for windows users (if you use Vista, you might want to install to another location than 'Program Files'):
