Monitor Turns Black After 5 Seconds

All of a sudden this problem started happening 2 weeks ago. I've noticed some unusual "humming" noises (like the computer is working harder?) It's an older computer… HP Custom Built 2005 model. Windows XP. Computer has been a work horse for many years. Only have 2 GB of RAM. Every time you power up the computer once it gets to the Desktop screen… the monitor goes black, dark, off… whatever you want to call it. I tried clicking the on/off button on the monitor and it never comes back on… until you do a complete shut down or restart. Then, it stays on for 5 seconds and goes off again. I thought it was a malware or adware issue so… I did a Full Format and Recovery. It worked well for about a week. The "humming" noises are still evident… but just last night it started the "monitor off in 5 seconds" again. I looked at the control panel… system menu.Windows claims it's running 2GB of RAM. I thought it was 4GB. But apparently in 2005… that wasn't the case? I figured the Ram was going bad? So, I opened up the computer… and sure enough… there are 4 slots for memory… just 2 used slots. Each stick says 1GB. So apparently the RAM isn't an issue. What possibly could be causing this? I really like this computer a lot. I remember spending $2,800 on it in 2005. I'm not advanced at all when it comes to computers… but I can follow step by step directions. If anyone could help… much appreciated!

i had a similar sounding problem with my daughters pc, and it turned out to be a failing power supply unit (psu) so that's where i'd be looking first on your pc for the problem, but to be honest because of it's age i'd be wary of spending any/much money on it, you could get a new pc for under $500 that would blow it away performance wise.

The screen's backlight inverter could be on it's way out or as Terry said, it may be the PSU. Try swapping in a spare PSU or hooking up another screen if you can lay your hands on either.