If you want to be able to leave the WkEnding text box blank upon opening the form, then you are going to have to wrap your DLookup in an IIf statement. It would look like;
=IIf(Nz([Forms]![Form1]![txtWeekEnding],"")<>"",Nz(DLookUp("PayIns","tblDepositInformation"," WkEnding = #" &...