This article lists some limitations in 64-bit Windows.
No Mixed 64-Bit/32-Bit Processes
64-bit programs cannot load and call 32-bit MDAC.
64-bit Microsoft Internet Explorer cannot load 32-bit ActiveX controls.
The 64-bit shell cannot load 32-bit Inproc shell extensions.
32-bit installer programs cannot load and register 64-bit DLLs.
No 16-Bit Code
No 16-bit code can run, except for recognized InstallShield and Acme installers (these are hard-coded in Wow64 to allow them to work).
16-bit Setup bootstraps are not supported.
16-bit MS-DOS and Microsoft Windows 3.x utilities will not start. If you attempt to start such a program, you receive a "Program.exe is not a valid Win32 application" error message.
No OS/2 or Posix Program Support
There is no support for OS/2 or Posix programs.
No Kernel-Mode 32-Bit Code
There is no support for Kernel-mode or 32-bit code such as:
32-bit virus-detection or 32-bit file system filters.
32-bit video adapter or 32-bit network adapter drivers.
32-bit Kernel-mode printer drivers.
The third-party products that this article discusses are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, regarding the performance or reliability of these products.