xp and windows 7 screwed inspiron 9400

so far on a old inspirion 9400 i have tried xp and windows 7, xp works but occasionally the whole screen (even on the dell boot menu) gets very dim and i need a flash light to see anything, on windows 7, i tried installing the driver from driverpack 14 and windows update, but that also resulted in a dim screen (this time not on the boot menu etc). I know it works on windows 7 since it was once on it and fully working. right now im writing from xp.. interestingly enough when i took out the battery and plugged it in it worked... but i need it to work just like that...
preferablely i need windows 7 on it
Did you notice any yellow mark in device manager or check your screen settings and post the results... i suggest you any other solution...
Dell Inspiron 9400 Notebook PC (support site)

Its support site has device drivers for Windows XP 32-bit and Windows Vista 32/64-bit.

If you install Windows 7 32/64-bit in it, you will need to search for its device drivers (chipset, graphics, audio, ethernet, wireless, etc.) elsewhere.

What's the exact 7-character service tag number on yours?
didn't realize how old this thread was, just peaked my interest because i also have an Inspiron 9400. I got the Dell provided Vista x64 audio driver and a few others to work on Windows 8.1 and Windows 10. You just have to install it manually because it will automatically adjust the compatibility mode. Without it it installed and the driver worked just fine.