Hi Katie,
Here is the HP support page for that PC:
http://h10025.www1.hp.com/ewfrf/wc/p...lang=en&cc=us&
Sounds like there still may be something unplugged or a conflict in the system. Put the original HD back in (but intially don't plug the IDE cable - 2 in wide - or power cable, remove everything you added to it - like the new memory. Next, write down on some paper where every single wire from the power supply goes to devices and to the motherboard. then remove power to everything but the critical like the motherboard. Remove and reset the motherboard connector, make sure the video card is tight. Try to power up the computer, see if the lights come on, CPU fan and Power supply fan (if they have them), any beeps, monitor lights up and show BIOS info. If you start to get things working, you can add items one at a timne to see if something is defective OR if it all happens to work the second time thru...
What operating system do you have?
Is this your primary computer?
Thanks,
John