DO NOT USE THE AFTERUPDATE EVENT OF THE FORM TO MODIFY THE CURRENT RECORD! IT CAUSES A NEVER ENDING LOOP.
Was that emphatic enough? I didn't equivocate did I. Use the BeforeUpdate event. The BeforeUpdate event is the LAST event fired before the current record is saved. The AfterUpdate event is the first event fired AFTER the current record is saved. Putting code in the AfterUpdate event that dirties the current record, causes Access to have to execute the BeforeUpdate event again, save the record, execute the AfterUpdate event, which now dirties the record causing Access to have to execute the BeforeUpdate event, save the record, execute the AfterUpdate event.... Are you getting the picture?