You may have to manually adjust BIOS settings for your machine. Others require a program on a floppy drive. Consult your user's guide for details. The final hurdle, if you get a large HD, is your computer may not be able to see the entire drive. Almost all DOS and Windows machines have an 8.4 gig limitation.
Many machines have hardware limitations that are even lower.
The easiest way around this problem is to download a BIOS extending utility such as MaxBlast. You should be able to download this kind of program for free from your HD manufacturer's Web site. The top-rated hard drive manufacturer's currently are Maxtor, Quantum, Western Digital and Seagate. Go with one of these and you can be assured a quality drive. My personal preference is the Maxtor Diamond Supra series, but I have also been happy with Quantum and Western Digital drives.