List of 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.

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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.

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No OS/2 or Posix Program Support

There is no support for OS/2 or Posix programs.

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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.

Reference: http://support.microsoft.com/kb/282423