Windows Vista x64 can compile to both 32 or 64 bits, and run both them. The compilation depends on from where the compiler is started : from "x:/Program Files/Microsoft Visual Studio 8" or from "x:/Program Files (x86)/Microsoft Visual Studio 8". These are 2 different directories that cvontain 2 versions of the compiler. When an user enters bug report to Bugzilla, neither Platform nor OS can specify which compiler was used. I never tried Windows XP X64, but likely it has same problem.
The OS and platform lists are hard coded in the bugzilla version we're using, and can't easily be changed. Just record if you're using a 64 bit executable in the description or comments. We have been doing this for some time with the linux builds, which have the same flexibility. If you like, remind me after we migrate to an up-to-date bugzilla and I'll see if it's easier to change there.
Wine's bugzilla has a "PC-x86-64" option: http://bugs.winehq.org/enter_bug.cgi?product=Wine (I seem to remember being able to choose that in some bugs I file, and wine is just another place I frequent lately; I don't know if the wine people customize it, nor if there might be other bugzilla place I go to where I have seen this).
We have a word size option in our Bugzilla