remove windows xp

Hello. this is really a stupid question that can be solved by looking in the userhandbook. but I dont have it!! and that's why i'm hoping someone can tell me: HOW do I remove the XP operating system from my inspiron 5150? I have done it before, but I dont remember how to start... I dont want to delete it from windows, I want to do it in dos, and I remember when I did it the last time I had to be real fast to press a button on the tabs before the windows flag came up. but WHAT button did i press? I also remember that after that iI could decide to format the HD. all this without putting in any disc. does anyone know what i'm talking about?? i really need to uninstall my computer now, and I want to do it the way I did it last time. please help me....