You cold set up a macro and call it autoexec (this always runs as soon as the database opens). Set up an option to openform in the Action section of the macro sheet, then type in the name of your log in form in the section that says "Form Name".
This works quite well, but is a much more long winded way of doing things. I only mention it to point out that if you need long reams of code to run, or something else to happen it can be done form the autoexec macro. You can , of course call code and then open a form this way.