Cannot open PDF files get "internal error occurred." Cannot Unitnstall Adobe Reader get "Error 1606.." Can't install Adobe Reader get "The following application(s) could not be installed." Tried to install from MS and Firefox. I have Windows Vista Home Premium
Regarding error 1606 - have a look at this article http://support.microsoft.com/kb/886549/.
If you still cannot uninstall Adobe Reader, use the Windows Installer Cleanup Utility to remove it. (But I recently heard of problems when using the cleanup tool if Office 2007 is installed.)
After removing Adobe Reader, download http://ardownload.adobe.com/pub/adobe/reader/win/9.x/9.3/enu/AdbeRdr93 0_en_US.msi, save it to disk, then close all browser windows and run the downloaded installer.