outlook 2002-2003 display the Msg with strainge characters

Dear all,
let me show my problem. i have 100 pcs with liecensd windows 2000, my
exchange server also is 2000, i want to upgrade the OSs gradually to Windows
XP, and i have done that with 50 PCs at the moments,my problem that when i
send any email through internal outlook from any machine to another with
windows xp the message apear to reciever in unreadable characters, and vise
virsa, i have doubts that the problem is regarding to the exchange server
because it is 2000 and the emails are sending to another version of OS.please
advise in this matter .