I have Vista x64 installed on my system. For some reason, Windows has dropped some of my default file associations and won't let me change them back to their original configuration.
For instance, if I right click a .doc file and select "Open with..." then "Select a program from a list of installed programs" Microsoft Word is not listed in the list of programs. If I click on "Browse..." and select Word manually, I'm returned to the list of programs from which to to choose, but Word still isn't available.
I've also tried setting file associations through the control panel. Again, in the "Set Associations" window, if I click on "Change program..." I experience the same issue.
The issue is not limited to .doc files. For instance, Vista reset my .txt file association to Vista's Notepad, however; I previuosly had the default file assocation set to Notepad++. When I try change the default file association back to Notepad++ I experience the same issue as for the .doc file type.