If at all possible, it is reccomended that you download the client before buying - you get a personal download link when you purchase the game, but if you test that it installs and runs now, then you will have less to worry about when you buy it.

The client is inside a self-extracting ZIP. Extract it, then run the installer, THEN run the game and let it fully patch up (click the ?UC?? button to patch/run the game), and then test that the client launches before buying.

Download the client:

A mirror kindly donated here from 3.

This is a torrent for the client, hosted on thepiratebay.org: Here!. The seeder is Warren Black/Warning Robo, so give him a shout in the IRC channel if the file needs a seed. (Unzip the file and click 'autorun' to install the game.)