VBScript read environment variable in if statement

What changes must I make for the if statement in my code extract below to work as expected?

dim warn, txtfilelocate, txtcontent, file, txtfile, content

call START
sub START
   do while len(txtfilelocate) = 0
   txtfilelocate = inputbox ("Please enter the full path of the text file containing your email content", "CDO Email Sender")
      if isempty(txtfilelocate) then
      wscript.quit()
      end if
   loop
end sub

txtcontent = DQ(txtfilelocate)
wscript.echo txtcontent

set file = createobject("scripting.filesystemobject")

   if file.fileexists(txtcontent) then
      set txtfile = file.opentextfile(txtcontent, 1)
      content = txtfile.readall
      wscript.echo content
      txtfile.close
      call HELLO
   else
      warn = msgbox(txtcontent & " was not found", 0, "CDO Email Sender")
      call START
   end if

sub HELLO
   wscript.echo "hello"
end sub

function DQ(str)
   DQ = chr(34) & str & chr(34)
end function

I suspect it has something to do with expanding environment variables, but as a novice I would appreciate clarification on what I'm doing wrong, and what I should be doing for this to work. Thanks.