XP Computer locks up with a solid black, yellow, green, or blue screen with no text. Must be powered down.?

I have a dell desktop pc running windows xp pro and it fairly often (daily) will completely lock up and most of the time is a solid black screen. Sometimes it is mustard yellow, bright green, blue or even solid white. Never any text on the screen, numlock is still lit up and won't change, caps lock won't enable, ctrl alt del does nothing. I think even power button stops responding and it has to have power cord unplugged. It had a power supply replaced recently and was doing this before and after, have checked and changed ram out. Windows already is up to date but I also did bios updates and everything available from Dell. Does this sound like a video card problem or cable? I would think it would be windows since the monitor works it just seems like output to monitor is bad, and with keyboard not working make me think it is software related? I'm thinking do fresh install of windows and try that but grabbing at straws to see if someone has had this happen and knows better than me. thanks! let me know if you need other info. also using avast and no problems with viruses lately. also used combofix, and malwarebytes that found nothing. also it will do this under load with programs going, or only logged into windows with no programs open it will do it.