How I fixed PDF file associations
I had this problem of Acrobat PDF files not being recognized in Windows after installing a Boot Camp partition to run Windows XP. I set up the boot camp partition, and set up my Parallels 4.0 virtual machine to use the boot camp partition for the hard disk. Then, I installed Adobe Creative Suite 3 Web Premium, which includes Acrobat 8. As part of the installation, I got updates from Adobe Updates (over the web), which were installed. After all this was done, I noticed that PDF files had some generic icon and double-clicking them would give a message that Windows didn't know what program to run.
Here's how I solved it, based on the information from John@Parallels and other web sources. The correct icons for PDF files were restored, and they open correctly when double clicked. I had Adobe Acrobat 8 Professional version 8.1.3 and Adobe Reader 9 version 9.1.0 installed. Although I have Parallels 4.0 installed on my Macbook, I did this all after booting natively to Windows from the Boot Camp partition. In other words, Mac OS X wasn't running.
If you are new to editing the Windows Registry, I recommend you back up your system just in case a mistake is made.
1) Run REGEDIT (Start, Run, regedit).
2) Find the key HKEY_CLASSES_ROOT\.pdf
An easy way to do this is to click on the top My Computer item in the left pane, and choose from the menu Edit, Find. Type in .pdf in the box. It should take you to this key. Be sure to confirm you have found key "HKEY_CLASSES_ROOT\.pdf" which will be displayed in the status bar at the bottom of the Rededit window.
3) Edit / Rename this key to .pdfbad or something or, if you are brave, delete this .pdf key (including its sub-keys).
4) Run your Acrobat or Adobe Reader program.
5) Note: after this step you will have to restart Windows. From the program menu, select Help / Repair Acrobat Installation (or Repair Adobe Reader Installation). It will run for a while. It will allow you to restart after it is finished.
6) Restart Windows.
To change which program is the default when double-clicked (I have both Acrobat and Reader):
Right click on a PDF file in Windows Explorer.
Click Properties.
If the program shown in Opens With is not the program you want, press the Change button.
Select Acrobat or Reader, or another program as you wish.
Verify that the checkbox "Always use the selected program to open this kind of file" is checked.
Press OK.
If all is working OK, you can clean up by running Regedit again and deleting the renamed .pdfbad key.
I hope this helps someone.
-Bob