About the only thing you can do with the text is you can bold the top line [section] of your message box using the EVAL function and the @ symbol. Ensure you keep the single quotes and @ exactly like my example. This will work for all versions of Access.
If (Eval("MsgBox ('Your Bold Text Here.@Your Normal Text Here.@@',20,' Your Title Here')")) = vbYes Then
MsgBox "user clicked YES"
Else
MsgBox "user clicked NO"
End If
You will have to play with the aurgument numbers [20] if you need a different symbol and/or buttons.
I think its always better to use the Access constants provided as its easier for people to read (vbExlamamation, vbQuestion, vbCritical, vbInformation) the number these enumerations point to.
Even better to make your own Public Constants in a module that tie all your possible combinations together.
Code:
Public Const mbQuestion = vbQuestion + vbYesNo
Public Const mbConfirm = vbExclamation + vbOkCancel