If you Key is not written on the laptop itself (like on the bottom) then it's embedded in the BIOS. If you can't recover the key from that installation, then you'll have difficulty getting that key back unless you acquire original Acer Installation media. If they key is outside, then you should be able to install any matching edition and install it. Activation could occur automatically if its in the BIOS, or you may need to furnish that key.
Of course, if you're buying a new and legitimate copy of Windows, then you can just use the key that comes with that instead of your OEM key.