Problems with Video on Dell Dimension E521

This is going to be a long post, so sorry in advance.  This is a Dell Dimension E521 running Windows Vista.  (I have not had the guts yet to install the new Service Pack)

A few months back, while under warrenty, my video card went.  Not sure of the exact model number, but it was a Nvidia.  We were not getting anything displayed using the video card, but when we plugged into the intergrated video slot, it would work, so we ruled out the monitor being bad and replaced the video card.  All was well until a couple of days ago.  Of course, it all started out again 3 days after my warrenty expired.  Go figure.

It started one day.  I usually leave my computer on during the day.  When I came back to the computer a couple of hours later, I could not get it to wake up at all.  Mouse was not responding and keyboard would not respond.  Turning the cap lock on and off did not  show any light signals or nothing.  It used to do this once in a blue moon and all I would have to do is hold down on the power button to shut it off and reboot and it would be fine.  But not this time.  It would still give no signal, even though it appeared by sound that the computer booted. Many restarts, and still no video. I did not hear any beeps or anything and none of the diagnostic leds lit up like it did when the first video card went.  I tried resetting the video card, no avail.  I then plugged back into the intergrated video and I was able to recieve video.  Great I figure, just need a new video card.

I brought a new ATI Radeon HD2400Pro.  Installed the card and drivers and I had video.  Great, I think, it's working again.  Not so.  After about an hour or two of surfing the net, the computer acted like it went into sleep mode, even though I was using it at the time.  Once again, the mouse and keyboard are not responding.  Nothing is waking it up.  Monitor says "entering sleep mode"  power button on monitor goes yellow.  But atleast now, I can turn the computer off and back on again, and it will work great.  I have not installed any new programs (other than the video drivers) lately, and the only thing that has been updated near the time this started was definitions for Windows Defender.  Plugging into the Intergrated, it works fine.  But I prefer to use a Video card rather than intergrated.  Kids play GTA San Andreas, and before all this started the graphics were great and everything was fast.  Now that these problems arise, everything is so slow, it's impossible to get away from the cops!!!

I have tried to run a virus scan to ensure there is not a virus causing this, but I cannot get it to stay on long enough to finish off.  After shutting down and restarting, I go back and check the scanner logs and it says nothing was found, but scan was not completed.

I am getting error messages under the Problem Reports, but it's all Greek to me.  I've copied and pasted here.  I just don't know where to go from here.  I've built a computer before, and I'm constantly fixing others, but this one has me stumped.  I don't want to replace the motherboard until I've ruled every other thing out first.  If you need more information, please feel free to ask.