Technically, this can be done. But legally, I doubt it. The Win7 license MUST be a "retail" license - one purchased separately at a retail outlet and not an OEM or upgrade version that came with or was purchase for another computer (like your old XP computer). And it can only be used on one computer - not yours and the wife's.
A "retail" license is the ONLY license that can be transfered to another computer. OEM and upgrade licenses can not, under any circumstances, be transfered to a new computer. Those licenses are tied directly to the "original equipment" for which they were purchased for.
If, and only if the Windows 7 license is a full "retail" version, then if this were me, I would use a removable drive bay. When the wife was using the machine, you slide her drive into the boot drive position. When you are using the machine, you remove her drive, and slide your drive into the machine (shutting down and unplugging from the wall before each swap). In this way, the two operating system do not interfere with each other, or the boot process. And only one OS is installed at a time. Then when the new computer arrives, you permanently install the Windows 7 disk into the new machine.
Note when the new computer arrives, it would be best to start over and reinstall Win7 from scratch because just putting the Win7 drive into the new computer will likely cause Win7 to choke when it sees a different motherboard, which is really many different hardware devices in one.