TextBox
I'm not sure what 'x' stands for.
If it is the value of the texbox you can do the following.
In the After Update Event of the textbox put something like this:
If Me.textbox > 0 Then
Me.textbox.FontBold = 1
Else
Me.textbox.FontBold = 0
End If
'replace textbox with the actual name of the box.