I managed to install adobe reader in Windows XP mode and PDF documents were opened seperate, this week I finally found the larger download file. And that works again, but nevertheless I find this a weird problem. As you can see nothing happens the download file name get's another file name with _1 and _1_1 and so fort behind it.
You may want to try the following:
Run this fix from Microsoft: http://support.microsoft.com/mats/Program_Install_and_Uninstall
Uninstall Reader using Add/Remove Programs
If you want to reinstall Reader, download it from http://get.adobe.com/reader/direct/