I tried to install Exchange 2003 on a Windows 2003 Server x64 machine. I was not sure if it was supported, so I gave it a try.All went well, forestprep, sysprep, then install Exchange, setup reminds you that IIS parts, NNTP and SMTP are missing, so I go ahead and add the missing components. There is where I got stuck, there is no ASP.net to select on a x64 machine! I could install ASP.net beta but I wasnt sure that Exchange would accept or even see that.So I did some searching and come across an article from winITpro:

No. Microsoft doesn't support Exchange 2003 on Windows 2003 x64 Editions and is unlikely to do so in the future. The current release of Exchange 2003 won't even run on the Windows 32–on–Windows-64 (WOW64) subsystem, which allows many other 32-bit applications to run on 64-bit systems.

I know you're probably disappointed to learn this news, but a sound technical reason exists. Exchange 2003 and Exchange 2000 Server depend on the Exchange Installable File System (ExIFS) driver, a 32-bit piece of code that runs in kernel mode. For Windows 2003 x64 Editions, all kernel-mode code must be native 64-bit. ExIFS isn't, and Microsoft isn't planning to spend the necessary resources to make it so. . . .Wel we have to wait for Exchange 12 which will ship 64bit natively (on DVD only)