I have a: Play.com USB Dual PS2 to PC Converter XP made by Joytech
It is pretty old and the driver CD is for XP. It sometimes works in 32-bit with EMS 32-bit Vista beta drivers.
Back to 64-bit:
The gamepads are not shown in the game controllers dialog. They work when programming using the SDL library as well as in an old game from 1999 called Re-Volt. I have downloaded the Windows Driver Development kit but find the documentation regarding game controllers lacking so far.
Are there any good resources on the net for writing such a driver?
Since you probably have no documentation on the protocol the converter uses -- manufacturers practically never release that sort of thing -- you're looking at a very difficult task. Unless you're after an "educational" (read: incredibly frustrating) experience, your best bet will be to just buy a new converter which is better supported by current OSes.
If you want to take it on anyway, though, your first step will probably be to get a logic analyzer and read up on how USB works. Good luck... you'll need it.