Hi, this might sound silly, but, have you got usb peripherals connected? If you have, disconnect them all and then try to boot up again. I had a similar problem once and quite by chance I discovered this worked for me. After mine started ok, I was able to reconnect all my usb items without any problems. I experiment quite a lot, and this answer has come to my own aid at least twice - and worked for me each time.
Or - possibly norton might be the culprit.
Good luck anyway ... Cheers.