Here is what I did to fix this:
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* Uninstalled every Adobe software on my computer in the Control Panel (everything; Adobe Acrobat, AIR, Flash, etc.). Flash Builder could not be uninstalled though (got an error telling me to use the CS5 Cleaner Tool)
* Used the CS5 Clean Tool and selected "Clean All"http://www.adobe.com/support/contact/cs5clean.html
* Using cygwin I deleted the following folders:
- C:\Program Files (x86)\Adobe\
- C:\Program Files (x86)\Common Files\Adobe
- C:\Users\<myusername>\AppData\Local\Adobe
* Rebooted
* Installed CS5 Master Collection Trial. Was able to input my existing FB SN there, so I'm running FB commercial and Flash Pro trial which was what I wanted.
Thanks a lot to Chris & his colleagues for walking me through this :-)