I am trying to change a text file in the Windows 7 C:\Windows directory (model.txt)

I need to spoof a model number to install an accessory that the stupid installer checks. (It is a Sony Vaio USB Dock that will only install on a Sony, but it was going fine on the old Lenovo that has just been replaced).

However, everytime I change the model.txt and save it (saves OK,I can reopen it) it changes back to the original after about 30 seconds, or when I run the installer.
It is obviously some sort of Windows 7 protection, but how do I turn it off??