Try replaceing just the .dll first.
If you have to re-install Win 7 and you've lost your disk, there are several tools available to recover the product key. I had to do that with my laptop's OS and Office. If your laptop came with Win 7 preinstalled, there may be a sticker on the bottom or back of your laptop with the key.
* disclaimer - these instructions are intented for users that have a legal copy of the software. :-)