If you are going to install it anyways then who cares. Remove whatever jacked up version you already have on there. Have you tried the Windows update instead of installing from Catalyst? If I am not mistaken you mind need to have at least 4.2 installed before you can install 4.5. Go to windows update and inistall any of the .net frame work updates then try again.
Edit. I think you need to have Win 7 service pack installed too before 4.5 will install.