What kind of pro disk is this? Retail or oem restore? 32 bit or 64 bit? If this is a 64 bit retail disk, you'll need to locate a 32 bit pro disk to install on the new machine. But you can use the old key since the old pc's died irrespective of 32 bit/ 64 bit. Also, if you have 4 gigs or more ram, install 64 bit to make full use of it.