The only thing I can think of is a hard drive incompatibility. Sounds strange, but it happens, albeit rarely. I've ran into it 3 times now in the last 4 years of making various builds. If the hdd fried but your computer did not suffer any other damage (ie if the the hdd failure was not caused by something like a power surge), than I suspect your the problem is that your new hdd may not get along well with the old Dell. Do you have an old hdd laying around? Anything 3.2 gigs and larger will allow you to install XP on it to see if it truly is a hdd compatibility issue between the new hdd and the old Dell mobo.
Finally, you're sure you've got all the optical drives and hard drives properly jumpered, right? Sometimes all it takes is to change things from Cable select to Master/Slave for things to start working correctly.
btw, that monitor incompatibility thing makes absolutely no sense at all.