If you use Windows XP or below, navigate to Start > Control Panel > Add or Remove Programs, select Adobe Reader from the Currently installed programs box and 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 application and then click Uninstall.
Alternatively, you can try using Revo Uninstaller. Even if it can't uninstall the application normally, it will force it to uninstall by wiping out all traces of its presence from the system.