Operating System Doesnt boot after swapping harddisk

let me say i have a Desktop pc[lets say pc a] with windows 7 installed at my hard disk ,I just brought another desktop pc[lets say pc b] wuth hardisk without operating system.

Now i am transferring just hard disks [a<-->b],why doesn't my desktop pc doesnt boot with windows 7 at my desktop pc b?

Is it there any technical reason for not booting[operating system doesnt load] in my machine b?

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