I have made a x64 build of PuTTY that will run natively on Windows XP
Professional x64 Edition and Windows Server 2003 x64 Edition.

Download installer executable here:

Download fixed source (no patches yet):

I mostly changed SetWindowLong to SetWindowLongPtr in the source code
which is required when building for x86_64.

This is only a Windows 64-bit build. No alterations or optimizations
has been made for 64-bit Linux.