I have a Compaq Presario that I acquired as a project computer.
When I received the computer, it was missing a hard drive, a case fan, and a power supply.
I subsequently installed a 300watt PSU, upgraded the RAM to 2GB, installed a 320GB SATA HDD, and installed a case fan.
I installed Ubuntu 12.04 on this computer and things seemed to be running great.

Recently, I would hit the power button, and the fans would start, but the monitor would indicate "no signal".
I removed the side panel and observed the system booting.  The following describes what the system is doing:

Upon startup, the CPU fan will spin-up for approximately five seconds then stop, monitor will have no signal.  However, the case fan will continue running.  No system BIOS information will be displayed on the monitor.  On rare occasions, if you leave the system running, the CPU fan will eventually start and the BIOS information will be displayed and Ubuntu will PROMPT for a login.

I've tried resetting the RAM modules --- no difference
I've tried a second 300watt PSU  ---- made my monitor go crazy
Checked voltage on original PSU ----- 5v outputs reading 5.14v (rock steady)
Tried a new CPU fan  --- shuts down after five seconds just like the original fan

Any help will be sincerely appreciated.Usually the 5 second shutdown is associated with either a thermal condition or cooling fan tach signal missing. Guessing you have thermal compound between CPU and Heatsink and that all fans are plugged in to the 3 pins.

Have you tried 1 stick of Ram or the Other to boot on a single stick of say 1GB. Usually RAM issues cause post "Beep" codes, but sometimes it really freaks out the main board. You have 2GB and said (Modules)

Also if you have a video card added to motherboard where motherboard has integrated video available. Boot it without the video card installed and VGA cable to integrated video. 300 watts is borderline for most systems using expansion video cards. I suggest 450 watts minimum for any systems with video cards added, especially if gaming.Thank you for the reply.

Sorry about the confusion, I have 2 slots for memory sticks and both are occupied w/ 1GB each.
I will TRY removing one and see if there is a difference.
No audible beep codes from the system during any of these tests.

This computer does not have an independent video adapter card installed.  It is running off of the SVGA port on the system motherboard.

Thanks.If onboard video is dead, the computer will fail to startup.  Onboard video also uses shared RAM.  If one of the sticks is bad, computer won't start.  Since it does start occasionally, it's likely the onboard video chip or the RAM is on the way out.  On more recent machines, the video is integrated into the chipset which have fixed heatsinks.  Are those fixed heatsinks in place?  Some times they break the mounting TABS & the heatsinks fall off or are just loose.  What is the model number of the Compaq Presario?Good evening harpo

If you are still trying to figure what may be wrong  your PC.

I have had to re-cap alot of older HP/Compaq's

If you wish shut down the PC and unplug it. Open the side of it and look inside the PC and right around the CPU bracket you will see a lot of round cans (capacitors) if you look at the top of them you will see a "+" and should be flat if they are domed and maybe what looks like a fluid and looks crusty like this Than you found your problem.

This is more than likely the issue. I am not saying it is but i have seen this a lot on older HP's/ Compaq's

Good luck, Mike    Thank you for all the constructive advice.
The computer is a Compaq Presario SR5000.

The capacitors seem to be clean and flat on top.
The RAM sticks in this machine consist of one 1GB Samsung and 1GB Corsair.
When I removed the older of the two (Samsung), the CPU fan began to spin, the BIOS ran and I obtained the Ubuntu login screen.  I left the machine at this login screen for about one hour.  When I returned, the login screen was still waiting for my input.  I supplied my password, the screen went black with a white arrow pointer on screen (pointer was responsive to mouse movement).  However, remained at this black screen.
Re-booted a few times and obtained the same result.
Eventually, system started behaving as before with the CPU fan shutting down after a brief spin at start and no BIOS displayed to monitor.

The monitor is fine.  I am typing this response at the same monitor with a Dell Dimension 2400 connected.
I found the original PSU that was in this Compaq -- it was rated at 250watt output.
I currently have a 300watt PSU installed in this box.

Thanks again for your help!Hi Again

Do you get any beep codes?

You removed one the memory sticks try shutting it down and switch them around and see how it does. (please shut down pc first and unplug ac cord)

Do you have a spare Power supply I know you said you have a 300 watt in it now. 

But from the way you are talking it sounds like you have 1 of 3 items that are bad

MoBo (motherboard / systemboard)
bad power supply
Maybe bad ram it seems that after you removed one it started to act better

How this helpsThanks hartbeatmr.
When the computer attempts to start, there are no audible beep codes.
I have tried switching the RAM around and found that by leaving the newer Corsair RAM in the slot closest to the CPU, the CPU fan continued to run and I obtained the Ubuntu login window.  However, after a few rounds, even this arrangement resulted in the CPU fan spinning and stopping and no image to the monitor.

I had a spare 300watt PSU (I was given it by a friend who indicated it was operational).  When I installed this PSU, my monitor displayed garbled video as if it was not sending the correct frequency.

I have some spare video cards.
I might give that a try also.

Thanks for all your insight.A little additional information.
This morning the system booted with some hesitation and I was able to run updates to Ubuntu.
When I was asked to restart the system, the problems re-surfaced.

Video was working from the onboard SVGA port.

I removed both sticks of RAM and booted the system.  The result was no beep codes and the CPU fan behaving as before with no information on the monitor.
An interesting side note was that before I removed the RAM and booted the system, I noticed that the CAPS Lock and Scroll Lock lights on my keyboard were flashing.  Not sure if this sheds any additional light on this problem.

Thanks.Finally ---- the solution!!!
Apparently, the onboard video component of my mobo is on the way out.
I installed a PCI video card in the system and set the BIOS to utilize PCI video.

Everything is working and I am typing this response from the system.
Thanks, again to all who lending their expertise to walk me through the possible CAUSES. Quote from: harpo on July 24, 2012, 05:31:48 PM

Finally ---- the solution!!!
Apparently, the onboard video component of my mobo is on the way out.
I installed a PCI video card in the system and set the BIOS to utilize PCI video...Everything is working and I am typing this response from the system...

...it's likely the onboard video chip or the RAM is on the way out...