There are a couple of ways you could do this. Your form could contain a subform. The subform is linked to the account text box of the main form. Then, when the user enters an account number the subform is automatically requeried. This is the way I would recommend.
If you want to popup a new form, I would initially Open the 2nd form with the visible property of the form set to False. Then, on the AfterUpdate event of the Account text box, I would filter the 2nd form (or reset its RowSource Property) and set it's visible property to True. (I would recommend using the filter way of doing it.) I would then leave the visible property set to True from that point on. The code to filter the 2nd form would be something like this:
frm2ndForm.Filter = "strAccountNumber = '" & txtAccount & "'"
frm2ndForm.FilterOn = True
To requery the 2nd form, type something like this:
frm2ndForm.RowSource = "Select * from YourTable Where strAccountNumber = '" & txtAccount & "';"