When you right-click a file and choose Open With, and click the Choose Program… option, the list of registered applications are displayed in the Open With dialog. Programs that are not already listed in the Open With dialog can be added manually using the Browse… option.
OpenWithAdd tool which I wrote, can be helpful in circumstances where, when you browse to locate an application, it does not get added to the Open With dialog. It can now (v1.2 onwards) run under Windows Vista as well. Here are some screenshots:
OpenWithAdd can be downloaded from this link.