Is this your own PC or more importantly do you have the proper user rights to install programs system updates etc and access registry?
If Yes, then:
Unable to update registry is the key error message which is quite possibly the reason why despite all your attempt, system fails to open .pdf files because the registry lacks the related info.
Before digging in to other areas, I'd advise you to check the:
Start > (Settings) Control Panel > Folder Options > File Types > .PDF
and check to see what program is associated with the .pdf files.
Quite possibly it is blank if system doesn't know what program to open that file type with. You will have to click on CHANGE > (from the opening window if Adobe Acrobat is not listed then) > BROWSE > locate and select: Adobe > Acrobat X.0 > Reader > AcroRd32.exe
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