...Doesn't work. It works before Windows XP has booted up (ie I switch to fullscreen on the Windows XP splash screen, real resolution switches to 640x480 or whatever the splashscreen is). Once Windows boots, it won't go lower, and going higher just puts it at 1280x800 (native). Core 2 Duo MacBook running 10.4.9 and the latest Parallels, with XP SP2 on a BootCamp partition.
