read the EULA
it states (not exact) you can have it on no more than two cpus
to be legit, you'll need to buy 2 copies, however this is VERY hard to enforce....
it really helps reading the Liscense Agreements... im one of those losers who actually read every single one when i install things. Its lke reading the constitution, you feel like you know what your rights are
My advice, buy 2 copies