Startup Problem 15R

Dell Inspiron Laptop 15R startup problem. On pressing the start button the power light and power indicator light flash white every 5 seconds. No hard drive activity screen remains black. Eventually computer will boot into Windows but might take morfe than 1/2 hour. Computer works perfectly when booted and will restart immediately but when turned off overnight the same problem manifests intself. I have updated to latest version of BIOS A10 and loaded all up to date device drivers.