On some new HP machines we bought, they come with pdf complete installed, which seemed to take over from Adobe 8. I removed them over the summer, loaded a pdf file and assocated the file with Adobe, and it works fine.
However, when a different user logs on, they get the "don't know file type" box, but if they select Adobe from the list it works fine.
Is there a way to assocate with Adobe across the domain rather for every user?
If you install/deploy Adobe Reader 9 (the latest Adobe Reader) this should automatically uninstall previous versions, but also re-creates the PDF file association.
I recommend you deploy the MSI on a computer basis, so the association applies to all users who logon to the machine.