There is some difference between the other machines and this computer.Here are a few things to look at:
1) Verify the same service paks are installed on this Win NT4 machine as the others.
2) Try it will same set of data on both machines. Just to prove it is not a data related issue.
3) Verify that the account pop up window is not over the drop down button. Move the window so the drop down button is not cover. (Note if you click on the account name and hold the mouse down it would also click on the drop down to pop up the list again.)
4) Try the same process but use different key strokes or mouse clicks such as just use the keyboard to select the account in the list and select ok(Alt+K). This will show if the problem is with "double clicking".
To answer your question, No there are not registry entries. There is an FNWIN.ini file in the windows folder. It holds settings for the workstation. Most of them are for windows size and position, column widths, etc. There is no setting about double clicking, etc.