You want to ensure that everything is setup correctly in the BIOS. If the HDD is connected, functional, and simply not detected during setup you may need to install a driver for the device.
Http://windowsreinstall.com provides you with step by step instructions to perform a reinstallation of the Operating System. In your particular case I'd recommend you look at the walk through for "no hard drive detected" at: http://www.windowsreinstall.com/winxppro/nohdd/indexfullpage.htm
My Laptop computer was pre-installed with Windows Vista Home Basic OS. Now when i try to change to Windows XP OS, the Computer does not recognise the HD, yet the same HD disk works very well. please help.