>> I need the user to complete the form once started.
there is simply no way to force them to complete the form. At the very least closing the browser, or turning off the machine will leave the form in an "incomplete" state.
What you really want is a way to tell the difference between complete, and incomplete records. This is, of course, a side effect of using auto-numbering, and that's been discussed before. (ie Autonumbering is bad).
Personally I use a "date/time started" field, and a "complete" field to identify errant records. The former are primed when the form opens, the latter when it is saved. A background process can then clean out old, incomplete, records.
cheers
Bruce