says...
Try this in the Debug window:

? Null = Null

THe result is Null, not True, which explains why you can't compare
anything to Null.

OTOH, there's a chance that your text box will look empty, but won't be
Null either (if you're bound to a field that allows zero-length strings).  
It's safer to check with a formula like this:

 If Len(Me!txtControl & "") > 0 Then
    ' You know it's not Null
 End If

That's generally how I do it... -- Ken

You can check a variable to see if it is null with the IsNull function.
Ken's solution is better if you want to check to see if your control
contains no data,  unless you somehow differentiate between nulls and
zero length strings.  

--
Tenbroeck G. Smith MA, Project Coordinator
Rollins School of Public Health - Emory University
1462 Clifton Road, Room 513
Atlanta, GA 30322
Work: (404) 727-8450.  Fax:  (404) 727-7261