If have GW3 follow the install instructions to create a second instance for GW3. Just name the .exe file to your liking
If I remember correctly there were two ways to install it. either copy your current install of FS9 to another folder or do a new install.
both methods have their own draw backs.
Copying the current install is gives you a duplicate of the out of control FS9 install you have now complete with any scenery updates you may have installed. Those scenery updates could be hard to root out. On the other side of the coin you would have the 9.1 updates because that version was updated.
Installing a fresh copy of FS9 gives you a clean install but is missing the 9.1 update. That update may not be able to run again as it will see that it has already been installed in the registry (I have heard this is the case but never tested it).
Third option and maybe the best.
Blow out FS9 completely and start over. Once you install FS9 and update it to 9.1 copy that folder to a new location to be a plain Jane install you do not touch.
You could burn it to a DVD the default folder is just under 4 GB. This will give you a base for when ever you want to do a new install/third or fourth install. Thats what I did on my last rebuild.
The down side of this is you have to download all your addons again...
But who doesn't love looking for FS Files