This was interesting and took some hours figuring out, stoke and me had some fun with it and whipped up a UDF.
 Code:
Function baseN2dec($value, $inBase)
  Dim $strValue, $i, $c, $y
  $strValue = CStr(UCase($value))
  For $i=1 To Len($strValue)
    $c = Asc(SubStr($strValue, $i, 1))
    If ($i > 1) 
      $y = CDBL($y) * $inBase
    EndIf
    If ($c < 65)
      $c = $c - 48
    Else
      $c = $c - 55
    EndIf
    $y = $y + $c
  Next
  $baseN2dec = $y
EndFunction

Dim $strServiceTag, $strExpresCode
$strServiceTag = "G51JIC1" ;modified this to a non-existant service tag
$strExpresCode = baseN2dec($strServiceTag,36)

? "Serial Number: " + $strServiceTag
? "Express Code: " + $strExpresCode