PowerShell is the main and very useful tool in MS Exchange 2007. How nice it would be if we can use PowerShell for Exchange 2003! Well, actually we can. There is a WMI class for Exchange 2003.
As easy as that
$Mailboxes = Get-WMIObject -namespace root\MicrosoftExchangeV2 -class Exchange_Mailbox -computer $ExchangeServerName
you can get the information about mailboxes from Exchange server 2003