Microsoft released Windows XP Professional x64 Edition as the first to work with x64 PCs. Windows XP Professional x64 Edition supports 128 gigabytes (GB) of RAM and 16 terabytes of virtual memory address space, as compared to 4 GB of both physical RAM and virtual memory address space for 32-bit Windows XP Professional.
With Windows XP Professional x64 Edition, you can run both 64-bit and 32-bit applications side by side. Your existing 32-bit applications run in WOW64, while the 64-bit applications run natively. This makes it easy to transition to 64-bit applications at your own pace. You can even run 32-bit and 64-bit versions of the same application in Windows XP Professional x64 Edition, allowing you to evaluate the new version of an existing application and transition at a controlled pace.
When moving to Windows XP Professional x64 Edition, there are some key differences. The most important is that there is no support for legacy MS-DOS and 16-bit applications. If you have an older application that runs in MS-DOS mode or is a 16-bit application, you shouldn't move to Windows XP Professional x64 Edition until you can either update or replace the application.
Applications that install their own hardware drivers will not work until there is a 64-bit driver available for them. Windows XP Professional x64 Edition doesn't support 32-bit drivers at all.