Javascript Overwriting current text values while checking for maximum character lenght

Hello,
I am working on an ASP page that has some javascript validation functions.
I have the following formula in the textboxes onkeypress value.

//checking that the textboxes input doesn't exceed the maximum allowed characters
function TestMaxlength(strNm,strtxtBox, MaxChr, e)
{
  var key
  if (window.event)
    key = event.keyCode;
  else
    key = e.which;
var MaxTxt = MaxChr + 1
var strTxt=document.form1.elements[strtxtBox].value
 if ( key == 8 )
    return;
  else
      if(strTxt.length > MaxChr)
            {      
            if (window.event) //IE
                  window.event.returnValue = null;        
            else //Firefox
                  e.preventDefault();    
                alert(strNm + " are limited to " + MaxTxt + " characters!");
                  MaxTxt = 0    
            }
}

Where:
strNm = the textbox field type i.e.  "Last Names"
strtxtBox = the actual textbox name i.e. "txtLastName"
MaxChr = the maximum number of characters for this text box i.e. "25"
e = event

My problem is that if I am tabbing and I need to udpate the existing value of a field where such value has already the max number of characters (like the State =2 or Zip Code = 5) I can't overwrite the existing value by just typing the new state.  I need to press the backspace or the delete key first to delete the current value.  Otherwise, I get my alert, i.e. "States are limited to 2 characters" because the 2 characters are already there.

I need to be able to start typing and overwriting the existing values if the text is highlighted even if the value is already the max number of characters. I'm assuming I need to check whether the current value is highlighted first and if it is, then I am supposed to delete it at the first key stroke (regardless if the first key is the backspace or the delete key).

Any ideas?

Thanks!