Answer
This question was answered on September 22, 1999.
Much of the information contained herein may have changed since posting.
Your system is giving you a very common symptom, in this I mean that there are more than a few possible solutions at work here. The video of a computer is basically the problem child of the group in that it is the one thing that is going to be affected by all the problems your computer will get.
First, a few safety rules. Make sure you are safely grounded before touching any computer component because the static charge produced by just walking on carpet floors can be enough to fry the equipment. Do this by (preferably) getting a static wrist strap, or touching the metal casing of the computer before touching a component.
Okay, first solution - Your memory modules might be the wrong type or are inserted wrong. Look in your motherboard manual or contact the Motherboard manufacturer (or you can go to the bios company's website) and find out what type of memory your system can take. To check the insertion nudge the memory side to side to see if it is loose.
Second solution - Hard drive setup. You say you just installed a hard drive. Make sure you have the right configuration, in this I mean the way the hard drive is set up. Dual, single, Master, CS. Look in your hard drive documentation on how to set up the little jumper settings on the back of your hard drive. This is crucial and in some bios will not let the system start if done wrong.
Third solution - Interfacing between board and hard drive. The interface cable with the red stripe on one side. In most common situations the red stripe will go closest to the first pin on both interface connections. First pins are usually marked out or in hard drive cases are the ones closest to the power supply. Once again check your documentation for more detail on this.
Fourth situation - Manual switch for onboard video. The situation you have in mind is a good possibility. In some cases, you have to see the MB manufacturer for the switch setting (look them up on the net) and just set it up from there.
Fifth situation - BIOS switch for onboard video. Borrow a friend's video card for a day. Put it in your system. Hook up your monitor to this video card and let the system boot. As soon as you see the prompt to hit del go ahead and do that and go into your BIOS setup utility. In there there might be an option to turn on your on-board video. sometimes going and restoring BIOS defaults (this option might be present) will fix it. then reconnect to your onboard video card.
Fixing this problem for now is a quick solution and will get you back on your way. But for the long run I recommend getting an independent video card because on-board components such as video and sound cards have not proven to be exactly the most reliable components. There are many problems that have come up with this configuration.
If these solutions do not work for you please feel free to write back.