Swapping Boot Drives Between Two Custom Built PCs

I have two computers in two different rooms that I built a few years ago. I want to move the main boot drive from one to the other and vice versa. I know this cannot be done by just plugging the drive into the other computer, so was hoping someone could point me in the right direction. I really want to avoid just wiping out and reinstalling everything. I tried searching but it seems the majority of people just want to swap a new hard drive into an existing computer rather than swap in one with already existing information. Any help would be appreciated.

As long as you still have the license information (key) for the computers, and the operating system types match, there's no reason you can't swap the drives between them.

If the machines are identical then you won't need to do anything special. Just make sure the BIOS settings match and all you'll have to do is reactivate the copy of Windows under the new hardware.

A warning for others, if you have a true RAID setup then that's a potentially different situation altogether.

For each system, open Device Manager and select the drive controller.
Sometimes the drive controller is listed under "IDE" or/and "RAID" or/and "Storage".
Sometimes you'll need to figure out which controller your boot drive is under; unless you want to preinstall for all of them.
Access the properties of the drive controllers via Right-click Properites or double-click.
Click on the Details tab, then select "Service". Mine for example says "iaStor".

Basically that tells me that iaStor service has to be installed and enabled on the system that the hardware will be receiving.
To check for it on the receiving system, navigate to "HKLM\System\CurrentControlSet\Services" and in my case, "iaStor". If the service is not present then I'll need to preinstall the controller software prior to moving the drive. If it's already there, then just look at the "Start" DWORD registry entry and make sure it is set to "0".

Sometimes device services from the old machine are listed under the "HKLM\System\CurrentConrolSet\Control\CriticalDeviceDatabase" key and in some cases, in which cases the system halts under the assumption that it won't run unless that driver/service successfully loaded. In that case removing it from the regkey then allows the system to load.

If you're just moving the system as opposed to the drive then it potentially gets more complicated as the volume IDs may have to be matched-up in order to avoid Stop 0x7b. There have been cases where I've had to check and enable CPU related services when going from an Intel to AMD machine. There was even a case where the Intel's CPU related service caused instability under the AMD based system.

You can try to see if the HD will boot up outright but it may require adaptation to changed hardware by SysPrep to move HD to another computer on the old hardware, or booting Paragon Adaptive Restore CD on the new hardware to P2P Adjust Win7 to the new hardware.

Have your Win7 bootable installer or System Repair Disk ready in case it needs Startup Repair, after if necessary confirming the System Reserved or C partition is still Partition Marked Active.

Either way it will when it starts change out most drivers in a cascade you can monitor by clicking the Driver-loading animated icon in System Tray, until all are finished and final Restart request.

Then enable Automatically deliver drivers via Windows Update (Step 3) and run all rounds of Important and Optional Windows Updates which may apply new drivers and patches Win7 wants.

Finally check Device Manager for any missing drivers, import from the Support Downloads webpage for your model PC or device.