i opened the bios battery for a minit and put it again and press F2 & F12 but its not work

I press F2 & F12 But bios not open.win 7 is installed in this laptop but i want to install win xp but it's boot menu not open.i opened the bios battery for a minit and put it again and press F2 & F12 but its not work . can anyone tell me what to do.''Turn on while tapping 'esc', then choose 'f2'...