How many times can you install Burning Crusade on different computers?

My friend recently bought the World of Warcraft expansion, The Burning Crusade. I was wondering if the software lets you install it on more than one computer.

Just to clarify what the posters above me were hinting at:

You may DOWNLOAD the software an unlimited amount of times, and upon an unlimited amount of computers. no question. (download, as in from blizzards web site) But the CD game itself has a single use secret key code that applys to one account alone. So if you upload the game from your friends CD, you will still have to buy a key code from Blizzard.

The issue lies with the account. Your friend could come over to your computer and play his game, his account, his character; by typing in his screename and password. After he leaves though, that program is 100% useless to you unless you own your own account.

hope to see you in game!

oh btw, dont share passwords, if Blizzard finds out, they usually ban the account