After the last round of windows updates, every single open file dialog in every single application exited instantly as soon as I click on the file type drop down!!
After Google'n for a bit (to no avail), I remembered this thread. Went back and deleted that damn .pc key in HKEY_CLASSES_ROOT, rebooted, and it's fixed....
I hadn't rebooted since adding that .pc key - so the windows update may of had nothing to do with it.