Code:
Dim buffer as string, lines() as string
'open the file binary
Open file for binary as 1
   buffer=space(lof(1))
   get#1,,buffer
   'split up the lines
   lines=split(buffer,vbcrlf)
   'modify the first line
   lines(0)="The line I want to insert into the file"
   'join the lines and put it back in the file
   put#1,1,join(lines,vbcrlf)
close 1