OK, this is wacky. Here is what it says up to where one downloads it:
Type: Driver - Universal Print Driver
Version: 5.0.3 (16 Dec 2009)
Operating System(s): Microsoft Windows 7 (32-bit), Microsoft Windows Server 2003, Microsoft Windows Server 2008 W32, Microsoft Windows Vista, Microsoft Windows XP
File name: upd-503-pcl5_winxp-vista.exe (14 MB)
Further down the page it says something about it will support 64 bit but I can't really make any sense of the rest of it. I'm not even certain it's talking about the same driver (HP's websites have always been obscure to me).
Whatever. I'll worry about it when I upgrade to WIN 7 (late this year at the earliest, if then). I have the driver book marked. If it works, fine. If not, I'll get a new printer (NOT an HP!); I've been considering getting a wide media laser anyway.