Help dual booting two Win 7 instances via software (if possible)

My apologies if this topic has been brought up before, I've been scouring the interwebs for days looking for an answer, and most don't quite answer the specific question I have.

I have two PC's custom built, both are Win 7 x64 machines. My wifes machine, which changed to the "kids" machine after we got her a laptop, unfortunately died (Mobo gave out). It's an older machine and not worth diagnosing and buying parts to fix, so I took the video card and HDD from it and expanded my machine (both GPU's are the same, so I can run Crossfire now).

My question is: I would like to dual boot Win7 via Software (if possible) or from boot (if no software option) from the existing Win7 install on the HDD. Currently I installed and can access the drive data, but I actually wanna boot from the old Win7 install from the old machine so the kids can still play "their" Win7 instance without disturbing mine. To be clear:

-These are two HDD's
-Each have a unique install of Win7 (individual keys)
-I want to access either of them via software (preferred) or boot (if required)

Thanks!
-Jeff

If the moved Win7 doesn't start as is likely when you try to trigger it's HD with the BIOS Boot menu key, boot Paragon Adaptive Restore CD and point it at the Win7 installation to P2P adjust it to start on different hardware.

If it fails to start then, run Startup Repair after confirming the 100mb System Reserved partition (preferred) or C is marked Active, with the other HD unplugged. Partition - Mark as Active (Method Two)

If these fail post back a screenshot of the maximized Disk mgmt drive map and listings with both HD's plugged in. Disk Management - Post a Screen Capture Image - Windows 7 Forums