You have to go into your CMOS settings and disable the BIOS shadowing... though my guess there is something else wrong as this setting has probably been enabled since you got the computer. BIOS shadowing isn't necessarily a bad thing... but there really isn't any advantage to having it enabled on a modern computer. Back in the stone age of computers, there was a definite advantage, but not anymore.
How old is the computer? Have you checked the hard drive to see if there are any issues with it? Slow start up and shutdown is usually indicitive of a software / malware issue... but it could also be a problem with the hard drive. You might also want to check your motherboard's capacitors to see if they are in good shape.