I have the same problem as the original poster. That is, this is a generic problem with all and any applications that I install, not a problem specific to one individual application that might be an upgrade to an older one, which is the problem most of the
people posting here have had.
This is a brand-new out-of-the-box Windows 7 x64 installation, Only one user, and has administrator rights. All other administrator-type things seem to work (uac, gpedit, change computer name etc).
So if I get a new application like 7-Zip and install it, the filetypes are not associated with it.
If I create a new account of type administrator and try it there, I get the same problem.
The reason I quote 7-Zip as an example (it happens with other applications too, like LibreOffice) is because 7-Zip is available as a 64-bit Windows program, and so my problem is slightly different from the OP's (see her point (2)).
Other things don't work, too. Dropbox can't change the file icon types or set the context menus. (I guess other programs can't change context menus either, I haven't checked yet).