I recently migrated some Windows platforms to Windows Server 2003 x64 which were running Windows 2003 Server (normal 32-bit version).
The reason for this was that i migrated to a new virtual platform (in my case Xen Enterprise 4.0.1) which supports 64bit Operating Systems, so it would be a quick win… If i didn’t had XEN (or another virtual platform), i would have to buy several complete new servers (hardware) when i wanted to do this (migrating to 64bit) because none of my ‘old’ Pentium III, Pentium IV and Xeon processors supported 64bit, even the system boards did’t support 64bit CPU’s.

I said ‘migrated’ before but i actually installed a new server (setup a new Virtual Machine in Xen and installed Windows 2003 Server x64). I then start ‘migrating’ the software/applications, which is i.e. IPSwitch Imail, Adobe Acrobat, IIS6 (Internet Information Services), Voxtron Axxium, database tools/applications like SQL Server or Desktop Engine and serval Windows services like Active Directory, DNS, DHCP, etc.
After installing the software i did a ‘freeze’ on the old machine (freeze = no changes on the data anymore) and copied the data to the new servers with the newly installed applications. Especially this last step would need some indepth knowledge of the specific applications and is different for every application, for some applications i even made a migration-plan or did the migration during the night.

Ipswitch IMail problems when using 64bit
For IPSwitch Imail 2006.x (sometimes called IMail 9.x) i could not make the External User Database working with SQL Server 2005 or Microsoft Access. I tried everything, when i looked at the task manager i saw the IMail processes running in 32bit mode, which means that IMail itself is not working at 64bit yet, maybe IMail 10 will ? All other (OS) processes were running 64bit on this x64 platform.

Then i realized it couldn’t probably make a connection because the ODBC Connection was created in x64 mode and not in 32bit mode. Because IMail runs at 32bit, it have to request the DSN at 32bit. Learning this.. you see a lot of other tools in the C:\Windows\SysWOW64\.
For me (in this situation) i used: odbcad32.exe (Microsoft ODBC Administrator) which lists different ODBC connections than C:\Windows\System32\odbcad32.exe does (this one lists the x64 ODBC connections when you are at a x64 platform). The name odbcad32.exe is a bit misleading here (shouldn’t it be odbcad64.exe by now ?)

BTW. Don’t try to open both (the 32bit and 64bit odbcad32.exe tools) at the same time, then something goes wrong, it opens only 1 time with the wrong settings.

Adobe Acrobat Reader problems when using 64bit
At one server i have installed Adobe Acrobat Reader which automatically prints PDF invoices to a network printer (a HP LaserJet). Since Adobe Reader 8.1.0, Windows XP/2003/Vista 64bit editions are supported. The print driver i used was also 64bit (because the print driver was installed on my Domain Controller which i had just migrated to Windows Server 2003 x64 and installed a x64 driver).
When i opened a .PDF document and tried to print it, Adobe Reader hang. It was clearly there was a bug in Adobe Reader 8.1.0 so i started searhing.. I saw this note which points to Microsoft KB #930627, saying there are some issues printing from a 32bit application on a 64-bit Operating System.

Note: It should be clear that Adobe said they support 64-bit Windows editions but Adobe Reader does not run in 64-bit mode, it’s only just compatible and runs at 32-bit.

Microsoft released a hotfix for this which can be requested by a simple call referencing the KB #930627. The Hotfix is not yet available by Automatic Updates (checked:10-01-2008 dmy) neither in Windows Server 2003 X64 Service Pack 2.

Scripts/Webpages using (32bit) Components
I had some scheduled tasks, which were self-written VBScripts. One script was using the ASPPDF component, another used a third-party Mail component.
After copying the Scheduled Tasks from the old system to the new x64 system, so the executable line (still) was: "C:\Windows\System32\cscript.exe c:\myscript.vbs", the script gave me several errors and didn’t work anymore.
I forgot the fact the ASPPDF component was 32bit and so my script needed to run in 32bit mode: "C:\Windows\SysWOW64\cscript.exe c:\myscript.vbs".
A few months later Persits made a special 64bit version of ASPPDF so i used that and ran my script in 64bit mode again.
Some vendors don’t have a 64bit version of their components or (if you’re lucky) their components run 32bit as well as 64bit.
Bottomline: Check wether your components are capable of running in 64bit mode.

Note: These problems can also occur when you’re running IIS in 64bit mode and have a (ASP) website using some 32bit components.
Change IIS to 32bit mode using this one-liner: "cscript C:\inetpub\adminscripts\adsutil.vbs SET W3SVC/AppPools/Enable32bitAppOnWin64 0"

Outlook Web Access for Microsoft Exchange 2007
OWA (Outlook Web Access) for Microsoft Exchange 2007 needs to run IIS in 64bit mode. Microsoft made a few mistakes by installing some 32bit components with Exchange 2007 (which will give you errors, offcourse, because IIS runs in 64bit). See KB #932439 for such a problem.