.Setfocus

I have a form containing a textbox where the user will enter a part number. In the textbox's Exit event I put some code that says:

Sheet1.Range("a2") = txtP1P1.Text 'sends P# to sheet
lblP1P1.Caption = Sheet1.Range("c2") & ", " & Sheet1.Range("d2") 'c2 & d2 hold a description if one's available
If lblP1P1.Caption = "" Then 'description is sent to this label; blank if no desc.
MsgBox "This part number is currently invalid. If you'd like to have this part added " _
& (chr13) & "please contact this program's developer..." 'msg if no desc. available
txtP1P1.SetFocus '??? returns to textbox????
End If

My problem is I can't get excel to set the focus back on the textbox with the bad partnumber. It just goes onto the next one. Any help?

Cory