You might find this guide helpful:
http://boards.cityofheroes.com/showthread.php?t=120140
The short version is: you can download on one machine and then copy it over to the new one by moving your entire c:\program files\city of heroes\ folder to the same spot on your new computer.
You just need to figure out the best way of moving about 3-3.5GB worth of game
Thank you Frost, that worked perfect 
Hello,
I was wondering if there's a way to download the game client on one machine in order to install the game on another machine.
The reason I ask is that I have a new computer arriving on Monday and I would like to download it now to save a little time.
It appears as though downloading it only downloads an executable that then downloads/installs it on the machine that has that executable. I guess I could just put that executable on the new machine but that doesn't save me much time with the actual download process.
I hope that makes sense, and please let me know if this should go in the technical forum instead. Thank you in advance for your assistance.