The BIOS is where all properly attached/functional hardware items...should first be recognized. If you hard drive is attached to the motherboard and it's not seen in the BIOS...things could be tough. If you have used this drive and it was properly seen before in XP, this should be one of the first things you check.
And...you should open the case and check to make sure that all cables are still properly connected.
System manufacturer and model?
Howstuffworks How BIOS Works - http://computer.howstuffworks.com/bios.htm/printable
How to access-enter Motherboard BIOS - http://michaelstevenstech.com/bios_manufacturer.htm
If the drive appears to be properly recognized on the main screen of the BIOS, then we have more work to do.