is this a new laptop? could be a faulty hdd. if you dont have the diagnostics utility for your hdd, check out the manufacturer's website of the harddrive maker and download the diagnostics utility and put it on a floppy. then bootup with the floppy, follow the onscreen instructions and test your hdd for any errors. remember to change the first boot device in the bios to "floppy" so the computer can boot to the floppy.
another thing you can do is to bootup with your windows xp cd if you have it. if you dont have it then change your first boot device to "cdrom" in the bios so the computer can boot to the windows xp cd. when the computer boots to the windows xp cd, choose repair in the main console. the computer will ask you for an admin password and just press enter to skip the process if you dont have one.
then on C: type in "fixmbr" and it should only take a sec. oh yeah type it in without the "". then restart the computer and see if this works. if you are familiar with opening up laptops, then try changing the hdd cable too with a spare one and see if that helps also, good luck.