how to access boot sector

I want to access boot sector and modify the boot progress , an example is to display my image on the black screen before the boot string show up (it's like the way you see the manufacturer logo right after turning on your PC).
I was told to use Assembly to do that, but I wonder if the .NET can handle it ??

no you can't . i think It is necessary to use native api

Native api's won't do it either. Search Msdn for how to write a GINA dll.