To be honest you would save MUCH head pain "PAIN" if you bought a new server and windows 2008 server........i would use to machines for many reasons.
You must have IIS (Internet Information Server) installed.
When you install IIS, you should select the typical install that includes an SMTP component.
Make sure your networking has DNS setup.
If you use DHCP, the DNS setting should be set, if you do not use DHCP then make sure that you have entered your DNS server names. Exchange Server will use your DNS settings when it sends e-mail to the Internet.
You must have Active Directory setup.
To setup active directory, go to Start -> Programs -> Administrative tools -> Configure your server.
Select Active Directory from the left hand panel, then select "Start" in the right hand panel to start the setup wizard.
Select "Domain controller for a new domain"
Take the default - Create a new domain tree
Create a new forest of domain trees
When it asks for your full DNS name - enter the name of your domain that you have registered. This would be the name to the right of the "@" for your email address, such as "YourCompany.com"
Accept the Netbios equivalent of your domain in our case "YourCompany"
Select the locations you want to store the databases
Accept the location of the system folder
On the permissions screen select what is appropriate for your network; if all machines are Windows 2003, then select the only Windows 2003 permissions. Otherwise use the default for compatibility.
Create a password to administer the directory.
Review your configuration and accept by pressing Next.
After pressing next - the Active Directory configuration wizard will spend several minutes creating the directory for you. When it is complete press finish and reboot the computer.