Hello and welcome to Bleepingcomputer.
There are a number of things that can cause problems with a computer booting up properly.
The list can be rather long one as to what is actually causing these problems with your system.
What are the beep codes that you are hearing when the system fails to boot up?
Here is what I would do, I would completely dismantle this system, if you are not fully knowledgeable with building a computer, I would seriously take notes and make diagrams of how everything was before you start taking the system apart, this way you have an idea how things were before hand.Warning: Before proceding with any attempts to repair or modify the inside of any computer, be sure to ground yourself to the computer's metal case before and at all times while you are performing this task.
The human body is capable of storing lethal amounts of static electricity, which is capable of destroying the sensitive electronic components which are located inside a computer.
If it is helpful take pictures, those will be very helpful as well in case you run into problems.
Now back to the dismantling part, I would remove the CPU from its slot, make sure the contacts look okay in the CPU slot, make sure the CPU looks okay.
If there is any old thermal grease on both the CPU header and heat sink, carefully remove that, I suggest using a quick drying cleaning solution such as using a house-hold alcohol such as isopropyl alcohol 70% and a clean rag.
Try not to bend any pins on the CPU or its slot!!
When you put the CPU back into its slot and it is locked down properly, make sure you add a bead of thermal grease to the CPU header, do not add much, too much is as bad as not having any at all!
The CPU header would be the highest part of the CPU that mates with the heat sink when it is installed.
Re-install the heat sink and make sure it is level, sometimes it happens that it looks level, but isn't, so please be very observant to this, double check it.
Make sure the CPU cooling fan is fully operational, spins a few cycles after spinning it with your finger, place it on the heat sink and make sure when you connect the CPU cooling fan's power plug to the motherboard you connect it to the one marked CPU_FAN, this is very important that is on the correct power port on the motherboard, there are sensors on the motherboard that are specific to the CPU fan speed pick up sensor built into this cooling fan.
Slots through out a computer can suffer from oxidation, they are made of metal and metal corrodes and oxidizes over time, this causes problems with electrical connections, it acts as a barrier, one moment the circuit works, the next moment it doesn't, over time it becomes persistent, the connection does not happen at all, the circuits involved then fail.
By dissembling the system, you are disturbing all possibilities of corrosion and oxidation build up, you are also eliminating the possibility of a loose connection.
While you are examining the motherboard, you need to observe the condition of the electrolytic capacitors, those are those large round cylinders that are located around the motherboard, if those have failed, or they are leaking, they loose their value and purpose and cause the circuit they are a part of to act erratically and eventual failure of the circuit.
These two links will provide you with enough information on what you should be looking for. http://www.capacitorlab.com/visible-failures/http://en.wikipedia.org/wiki/Capacitor_plague
There is a process we use in trouble shooting computers by which we remove the motherboard from the tower, this eliminates the possibility of the motherboard shorting out to the metal frame of the tower.
The motherboard is removed from the tower and put on a table where it has no contact with metal objects.
During this process, everything from the system is disconnected with the exception of the critical components to get the motherboard to fire up.
That would mean the motherboard would have the following necessities hooked up to it in order for it to boot up.
A system board can still boot without a hard drive attached to it, it will send a system error if you do not stop it from looking for a boot device, but if you enter the BIOS setup utility at POST, there won't be any POST errors at this point unless something is wrong with the motherboard.
This is how you determine a motherboard problem. Yet I want to mention, we have 3 things in this scenario that can cause a problem, the mouse, the keyboard or the video card can still cause issues at this point, the key is if you are getting errors here, usually the system will alert you that there is a keyboard, video card or possible mouse error.
If the system seems normal at this point, you shut the system down and add another piece of hardware, ONLY ADD ONE AT A TIME!
In this case we now want to add ONE stick of RAM to slot number one, if fail, try slot number two, if fail, try slot number 3.
If that stick fails in all of the slots, it is the RAM stick that is bad. Get rid of it!
If it works and no problems, DO NOT add more RAM yet, try adding the hard drive and set it up so you can boot to it, see how the system acts.
It might be slow due to the fact you are low on RAM, but at least you can test the system.
If all goes well, shut down and add stick number two, make sure you follow motherboard manual instructions, sometimes the RAM has to be installed in the correct slots or system errors can result!
Try adding more hardware until you either find a problem, to which you now found the culprit that was causing you the problems, or you may actually have your system completely re-assembled and find the problem has gone away completely and does not come back again.
That would mean you either had a corroded connection that is now corrected, or you had a loose connection that has been found and corrected.
Hope this helps.
Any questions or concerns...please feel free to ask.
Now it's time to send this up and look for the typos, I am sure I typed plenty of those as this is a long post!
Edited by MrBruce1959, 10 May 2011 - 01:19 AM.