Ant, they want you to buy two copies. In agreeing to their license conditions when you installed it, you doubtless agreed not to try to and "squeeze two pax into one seat".
It is legal
in some jurisdictions to make a backup copy of software you own. You will probably need a specialised piece of software to do this: most game CDs are equipped with some sort of anti-copying mechanism. See:
http://www.gamecopyworld.com