The 64-bit version of Windows is designed to make it possible for developers to use a single source-code base for their 32-bit and 64-bit applications. To a large extent, this is also true for 32-bit and 64-bit Windows drivers.
However, 32-bit drivers cannot run on 64-bit Windows and must be ported. For user-mode applications, 64-bit Windows includes WOW64, which enables 32-bit Windows applications to execute (with some loss of performance) on systems running 64-bit Windows. No equivalent translation layer exists for drivers.
For information about porting drivers to 64-bit Windows, see 64-Bit Issues in the Windows Driver Kit (WDK).