Form: texbox navigation

I have a form set up with excel (2003) using vba. I'm doing a check to see if textbox1 is empty, if it is, a msgbox pops up with a warning that "textbox1 needs to be filled out". I click OK and the cursor goes to textbox2. I want the cursor to return to texbox1 without user intervention but can't figure out how. I'm using "Private Sub TextBox1_Exit(ByVal Cancel As MSForms.ReturnBoolean)".
I know it can be done because I did it with vba Access, but I can't seem to find it using vba Excel. Any help would be appreciated.