Start-> Programs-> Administrative Tools->Manage Your Server and perform below mentioned steps:
1) Click on Add or remove a role. This will start the Configure Your Server Wizard. Read the text and make sure you have connected all the necessary cables and all the other things it says you should do before proceeding further.
2) Click on Next.
3) This wizard will now detect network settings. It will take a while depending on how many network connections. From here we can add/ remove roles for our server. We will add the Mail Server role. Read about mail servers and click next.
4) Click Mail server (POP3, SMTP) and Click Next.
5) One will now specify the type of authentication and type the email domain name and click next.
6) Next step is to confirm the options you have selected and click next.
7) The installation will start, and will also start the Windows Components Wizard. When get prompted to insert your Windows Server 2003 CD-ROM into your CD-ROM drive. You can now see the log and click Finish.