how did you switch from Gnome to KDE? If you have just chosen a different session type in the login screen (what is your distro, btw?), then the drivers should be the same. If you did some XF86Config type of thing (reconfiguring the X from console), then the driver info in the config file might have been overwritten. If this is the case, the drivers should be there, but the /etc/X11/XF86Config-4 (or xorg.conf) file has to be fixed. Try looking for something like XF86Config-4.old in this case. Or just re-install the drivers.