I don't know if there is a way to uninstall IE9 on a system where IE9 is the default built-in version.
In my experience installing an older version of a program on top of a newer (installed) version is asking for trouble.
You may create more problems by attempting to install IE8 on top of IE9.
Best case is installing IE8 works and you don't have any problems.
Next best case is the IE8 install will fail before making any changes, if the install won't work.
Worst case is the IE8 install will make changes before failing, leaving you with system problems.
What i would do before making any changes:
Create the system recovery discs using the manufacturer utility.
Create a System Backup Image using a program such as Macrium Reflect (free).
Create a restore point.
I make sure i have every recovery option available before attempting a change like this!
If you can't install IE8 on top of IE9, and the compatibility mode does not work you might consider this:
Find an older version of the Windows 7 ISO that has IE8 as the built-in browser.
Re-install Windows with that ISO/version.
My guess is you need a pre SP1 ISO...maybe the RTM version?
I don't know if/where you can get the valid/genuine ISO version with IE8 from Digital River.
If it was me, i would ask Gregrocker
about the possibility of getting and installing the correct ISO with IE8.
IMHO he is the best and most experienced installation expert here.