You have to reinstall everything.
There is actually way more difference in internals of Windows XP x64 (compared to 32-bit), than it was even from Windows 98.
There's no more 16-bit compatibility, and all 32-bit apps need to be installed under certain compatibility layers and emulators.
I strongly recommend you just dual-boot x64 and 32-bit XP (that's what I do). Some products do not yet have x64 drivers, and some programs don't work in x64 yet.