If you put the old drive back in does it do what it used to do or go black like this one? That may eliminate the hard drive as your problem.
If the old drive does work then the new one is probably hooked up wrong. How did you put the operating system on the new one? If you followed the instructions for copying files and system off your old drive onto your new one, did you remember to change any jumper from slave to master or alternatively move it to the primary connector if cable select. Did you leave the old drive connected? Lots of possible combinations here that you did not mention.
If you find that both drives don't work then you will need to remove/disconnect everything you can except the basics (MOBO, Power Supply, memory and video/monitor, Keyboard) See if it gets to the boot screen and into setup. If yes start adding things back in until it fails. When it fails it is either what you just added or a partially bad or underpowered power supply. First replace the power supply as it is usually the cheapest. If that doesn't work replace whatever caused it to fail. If it still fails than you do the next step.
If in a basic configuration it doesn't make it to the boot screen and into setup you will need to refer to the motherboard manual and see what diagnostics it has (usually blinking lights or beeps). The sequence of those will hopefully tell you if its power, bios, cpu, memory or CMOS.
There are many many posts up here offing other similar diagnostic procedures.
I have an hp Pavilion - 1.8 gig board
512 meg of ram - 80 gig harddrive and
is running win xp. I was having trouble
with The computer not wanting to boot
at times and just going blank here and
there. I figured is was probably the
harddrive, so I bought one and I have
now put it in. Now,lo and behold another
problem. The system locks up at boot time
(right after turning the power on.) I get
absolutly nothing. The computer is on
but all I get is a black screen. I am pretty
sure it is all hooked up right. I can't
even get to the logon screen to get to
the bios? Could someone Please , Please
help me with this. I am at my wits end.
I would appreciate any help I get.
Thanks much, Pajemar