Shut down upon reboot - even safe mode

Last night I was using the desktop Dimension 2350 just fine. This AM, it was off when I woke up, so I started it. It booted up, windows XP loaded, all icons on the desktop came in fine, then it just shut down. Numerous tries at that, same thing happened. Booted it normally tonight to notice if it seemed to shut down upon load of something in the tray, all I saw was the volume and then AVG load, then it shut down.

 

I tried booting in safe mode just now, and it shut down before Windows even loaded. It stops in the process of scrolling through that long list...

 

A month or so again, I had a few weird shutdowns, thought the fan was out, and it woudl shut down when it got hot. Then it worked for months. I can feel air coming from both the power supply and the fan, but it's not as noisy as it used to be, but hasnt been in the time it's been working since the shut down's a month ago.

 

I did run an microtrends housecall yesterday, but it ran for two hours after I let microtrends clean what it found. No problems whatsoever for the two hours I used it after the microtrends. Left it on when I went to bed, got up and it was off.

 

I ran the microtrends because i had been working on a word document, that suddenly jumped fom 20mg to 60mg. Thought that odd because I had only done minor format changes, certainly not added 40 MG's worth of anything. And yes, microtrends found a small virus or two, I chose delete but since it ran fine for a couple afterwards, I find it hard ot believe that might be the issue.

 

What would shut it down upon start, even in safe mode? Any thoughts appreciated. Wondering if I should just buy a fan, and any one know what kind I would need?

 

TOnight I tried booting into other users name...same effect, altho if I left it running while the name choices were up there, it stayed on quite a bit longer, but finally did, just shut off.