If you use Windows XP or below you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the Adobe Reader 7 program and then click Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the Adobe Reader 7 application and click on Uninstall.
After that, run a registry cleaner and try again to download and install the software. Also, you need to know that nowadays, the latest version of the software is Adobe Reader 11.0.08 which can be downloaded free of charge from the official web page.