Hi,
At a guess, I'd say your motherboard wont support a large hard drive. I expect your new one is 20+ Gb?
You may be able to resolve this by updating your BIOS if there is a new version for your motherboard, otherwise you may need to use a disk management utility such as
Disk Manager, EZ-Drive, or DrivePro
Check out the following Microsoft article for more information...
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q128730
(remove spaces from the above URL)
You'll find the BIOS upgrade on your motherboards manufacturer's webite. Or you may not... It sounds like a very old motherboard.
Failing all that, you may need to upgrade the motherboard. You'll also need new memory, a new CPU and a new case at a minimum if you go down that track so it may be worthwhile investing in a new system altogether.
Cheers,
Tim.