get an cpu & hook the drive to new cpu & install the os while working current machine use kvm switch to connect both cpu's with one monitor
or get VMWARE & create a virtual machine to use the second hard-drive
i don't know about other possibilities that you are talking about
IF you are going to boot by changing which drive boots out of BIOS, its no problem. Install on the new drive and go.
If you want 2 Windows7 bootable instances, you can do that.
If you want to run 2 versions of Windows at the same time, thats not as easy but can be done. Monojgj had a couple of suggestions on how to do that
In all cases you have to watch for how many times you activate that version of Windows. Its more than 2 but not much more...before it invalidates all copies. Happened to me at work when I wasnt paying attention to the disks...