Blue screen on boot up

Hi thee and thanx for your reply i have tried inserting the install disc before i boot up as well as after it has boot up but the same result occurs it just comes to the desktop with my mouse moving minus the icons and to answer the question about having installed any software im lead to beleive that my daughter inserted this music cd that comes with cereal packets somtimes and it sort of hasnt worked since then but if i insert the install cd before i boot up shouldnt it let me erase the hdd so i can install a new copy of OS x 10.2?
and is there any buttons i can press on the keyboard to go into menu's or anything like a bios like pc's?