Formatting c drive

My grandson has a dell computer, that had vista, and he loaded windows xp, and other things, now everthing comes up working ok with the memory. But all that will come up is the main screen of windows. Then it won't do anything else. I am not at home now but I need to format c: drive, I have all the disks, it tells me dirvers are missing. I have forgotten which function key to use when I want to get to DOS so that I can format the c:drive
I don't think that there is anything else that can be done. It will not take the disks as it is right now. I think its memory is confused.