ahhh...this is very easy
now go to this page:
Drivers - Download NVIDIA Drivers
and then download your nvidia card
mine is like this.
dell xps m1530
nvidia geforce 8600 m gt.
that is my card.
which I will choose:
Product type:geforce (I look at the title and I see that you are geforce so you choose geforce.)
how to know product type.it easy
look at my name card
nvidia GEFORCE 8600 m gt
the GEFORCE is the product type. if you are not geforce but it's nforce or something.just check the words that after the word name nvidia
product series:it's where you choose it's notebook(I think it's for laptop) has the M.you are 8400 M gs right.you choose Geforce 8M series
now came to product(it may have for some):you choose your name of your nvidia graphic cards.find the line it says: GeForce 8400M GS
For me,I'll choose GeForce 8600M GT/GeForce 8400M GT
"/" means or
but I'll choose GeForce 8600M GT because it may better than GeForce 8400M GT
and if you are desktop,you choose the product that is no letter M(it means for notebook)it just series.
now:
operating system:you choose your operating system.Now,you choose windows 7 and find you are 32-bit or 64-bit
how to know you are 32-bit or 64-bit:
This MAY work.
press start+run(hold the windows logo and then press R button to start the run)
press dxdiag
it may have a window open. you can press yes or no.
and then look for the operating system.you will see you are 32 or 64 bit
if you are windows 7 x86(32-bit) choose windows 7 32 bit
the other x64(64-bit) choose 64-bit
and then press download and then install normal