Inspiron 6000 with ATI X300 will only display on external video when using Dell drivers

So I have this system, it's 2nd hand to me. All seems to be Ok with it except one dimm slot is bad. Other slot works fine with 1gb ddr2 sodimm. The bios says it has an ATI Mobility X300 card with 32mb hypermemory and 15.4" SXGA+ screen. I loaded Windows XP Pro, all works fine (Windows default VGA driver will do 1280 x 1024 but it's not very good) until I load the Dell video driver and reboot at which point I get no LCD screen video. Bios is latest, all other XP drivers are installed.

Bios screen comes up fine, XP Pro starts but as soon as the progress bar starts across the screen it goes blank. External display is fine. When I look at adapter properties, the ATI options are there and I look at Displays, it says the LCD panel is "disconnected" with no option to connect.

I install Windows Vista Beta 2, all works fine, LCD panel is detected correctly and Aero is Ok. Vista recognizes the adapter as an ATI Mobility X300 with no extra drivers.

I've basically tried every ATI mobility X300 driver I can find with no effect. Other posts mention the Microsoft driver for this but I don't have that option for some reason.

Any ideas ????