I have tried this trick on an Acer Aspire and an HP Pavilion dm4 3000TX.
This guide assumes you have already extracted a Windows 7 ISO image onto a bootable USB drive.
Open the file sources\ei.cfg
You should see something like this
Change the Edition to whatever Edition your OEM key is for and change the Channel to OEM.
Go through the install and enter your key when asked.
Sometimes the activation works, other times you’ll need to use the phone option to activate.
You should now have an activated genuine installation of Windows again. Enjoy!