I am using outlook express6, with xp operating system. I formatted my system, and installed again xp o.s . So all mails are deleted, how can I retrieve my mails. I may not tick the option to leave a copy in the server. Help me...

Well if you are lucky there may still be a copy on your mail server.
This is the ONLY way you will get them back, as you formated your computer ALL information has been deleted.