I dunno about 2003, but I know there used to be a way you could do it via the registry. This was back with Exchange 5.5 and 2k though.
edit: yep:
1.run regedt32 from the command prompt
2.Goto HKEY_LOCAL_MACHINE hive
3.Goto SYSTEM\CurrentControlSet\Services\MSExchangeIMC\Parameters\Extension
4. Add value name of "OutboundAppend" (without the quotes)with type REG_SZ using Data value of "\par\par\b <YOUR DISCLAIMER message.......>\par\par\b0" (without the quotes)
N.B: \par\par\b.....\par\par\b0 is used to break the disclaimer into separate lines. You need to put \par at the
end of each line before continuing the disclamer else you will have one very long line.
Also, this is for 5.5, but there should be something similar for 2k3
Double edit:
Try this: it's not as simple as before, but saves you buying any 3rd party app.
http://www.redline-software.com/eng/support/articles/msexchange/2003/disclaimer_fun.php