Tom says it takes over a day for his computer to boot up. Leo says it sounds like the startup process in the hard drive is damaged. If he can take the hard drive out and put it into a new computer, like Tom did, then perhaps it's a cable or power supply issue. Tom should try booting from the USB key and see if it takes the same amount of time. If that works, then clearly there's a driver or software issue causing it to boot up slowly.
Leo recommends that he back up his data, format the hard drive, and then reinstall windows. Tom should also try unplugging all of his USB devices and see if it boots up faster.