Can't change the bios priority (vista)

Hello,

i bought a new computer from asus. 64 bit, 4 GB RAM. but with windows vista basic - 32 bit.
i want to install win 7 - 64 bit, but i cant do it. i've put the dvd and tried to get into the bios to change the priority but all i see there is one option: microsoft vista. i dont see any sign of the dvd option or other boot sequence. only the vista. because of that i cant install win 7 or format the windows vista.
what should i do???

thanks!!