preparing one time boot menu

Hi, I got the vostro 1520,recently, I´ve been having this problem, when I turn on the computer this phrase appears on the right upper side of the screen: "preparing one time boot menu" then it starts making this annoying sound and starts a BIOS screen with this choices: 1. hard drive. 3. cd/dvd 4:diagnostics. 5. removable devices. 6. network.     .

I´ve already tried all the choices but it keeps on appearing.

what should  I do??