Have you tried uninstalling Reader 9, then reinstalling it?
If you really want to install Reader 7, you can download it here http://ardownload.adobe.com/pub/adobe/reader/win/7x/7.1.0/enu/AdbeRdr710_en_US.exe
Uninstall Reader 9 before installing version 7. Be sure to update it to the latest version 7.1.4 afterwards. And please note that this version is no longer supported, i.e. Adobe is no longer issuing security fixes for Reader 7.x.