Dell dimension2400

I bought a dell dimension 2400 at a garage sale last week that was for parts i brought it home and found that the processor chip was out of its socket and broken i replaced it with another one that i had. The computer came with a western digital 80gb harddrive with xp home edition and now i want to add my seagate barracuda 7200.7 harddrive to it. I hooked everything up properly and now the system bios says that there is no harddrive installed at all. I take the seagate out and it finds the western digital just fine. I've tried just the seagate alone and it won't find it i know that both harddrive's are good cause i took the seagate out of my old computer cause the motherboard was shot. I've had the seagate in a couple of other computers and it finds it and it works just fine. the bios is A05, processor speed is 2.7ghz, it has a intel celeron 2.70 ghz cpu. what am i doing wrong that it won't find the seagate harddrive? I've tried other harddrive's in it and it finds all of them. Its just this one that it won't find.