I have HP Compaq nc4200 laptop. Has windows 7 on but I'm trying to install windows XP instead through bootable usb drive. But laptop isn't booting from the usb, i just get the "non system disk or disk error replace and strike any key when ready" message. I checked the bios booting selections many times, and the priority was for USB flash drive ( USB hard disk- as it says in bios boot options). Any help is appreciated.