OEM OS ......locked to the machine.
In your case Acer.
Now if you put that HD in the compaq unit and then loaded the compaq OS .....different issue.
I bought a new laptop (acer) from walmart this last winter. It was running win 7. The motherboard died. (long story) Hard drive was ok. Put the hard drive in a compaq laptop that I have that had a bad hard drive. Has been running just fine for months. Last night out of the blue it started saying that my copy of windows is not genuine and microsoft will no longer provide support or updates. While I have had troubles in the past with updates doing stupid things I'd rather like to keep my options open. I still have the old acer laptop so I have the key from sticker on bottom. Any ideas on how to tell windows to shut up and it is a "genuine" copy and all proper homage has been paid to Mr Gates?