In addition to the previous answer, look at the pin configuration and jumper position on your old hard drive, is it in Master mode? Or Cable Select? Instructions printed on the drive should tell you what to look for. If it is in Master mode, proceed as Dave has correctly suggested and attach the new hard drive to the ribbon in slave mode using the correct jumper settings.
If instead the first hard drive is set to Cable Select, set the new hard drive's jumper to Cable Select as well and simply attach to the next connection on your IDE ribbon cable.
After installation, you'll have to format the drive. I suggest using NTFS if you're using XP or Vista. If you're using a previous version of Windows, you'll need to use FAT32 instead.
Ryeguy, January 2009