I have a Dell Dimension 4600 which is about 3 yrs old.
P4 2.6 GB processor and 1.5 GB of Ram.
MOBO is Intel 845p

To start, I know my way around a PC and am comfortable making changes and upgrades on my own.

Lately I'll log onto my PC, be logged in for anywhere from 10 to 30 mins and then the PC will just shut down for no apparant reason. Weather I'm on the internet, playing a game, or just checking email.

The front LED on the tower will then flash green and not allow me to turn the pc back on.

I leave it alone for a few hours or until the next day and I'm able to boot the PC up again but will only get another 10-30 mins out of it once again.

I have done all of the following:

-Confirmed the fan is running on both the power supply and the video card.
-Changed out the power supply....same thing happens
-changed out the memory...same thing happens
-changed out the motherboard...same thing happens
-completely disconnected everything from the motherboard and reseated all of it....again same thing happens.

I've spoken to the morons at Dell Support, who have provided no help.

I can't imagine that 3 yrs is the life span on a PC these days.
Escpecially since I am still running a P3 700 MHZ w/ 256MB of ram with no problem. This PC is for basic use though, internet, email, word docs, etc.

I need my main pc up and running.

Please help.

The Dimension 4600 is a rugged computer that lasts a long time with little trouble. I see nothing in your description that presents hints of a problem.
We can assume that you have good, current spyware, adware, and antiviruses installed. We will also assume that your power supply and cpu fan are free of dust and hair. So you present a fairly unusual problem.
When you have this shut down, are there any blue screens or other error messages on the screen at the time, or when your turn it back on?
Three years is enough age to consider replacing the CMOS battery, and the CPU fan even though you have done a change-out of the fan.
Look for cables that are brittle, or stretched, then reseat them as well as all your PCI devices.It does sound like a hardware problem. Sometimes a pci device such as a modem or sound card will creat this problem. Sometimes a hard drive will have intermittent spurious responses.
Since you did not mention the hard drive, I would suspect that as the problem area. Hard drives often contain corrupted software after three years. It is often a good idea to save your data, then reformat and reinstall eveything. The Dell Dimension 4600 usually (but not always) used either a Maxtor or a Western Digital. The Maxtor was one that had early failures, and the WD hasn't been much better.
With hard drives now so inexpensive, with the Seagate ATA 500 Gb now going for $140 at many locations, and the Seagate 80 for under $60 while the 40 GB is $40. This problem might be a good excuse for change... The Seagate has a five year warranty. See www.zipzoomfly.com, www.directron.com, www.tigerdirect.com, www.outpost.com, www.pcconnection.com, or any other major site for good prices.

Thanks for the insight, RayBay. I should also mention that I did have to replace the power supply on this machine 1 month after the 1 year manufacturers warranty expired. Convenient. I found that the model and year I purchased this machine had known issue with the power supply. Since then I've had 2 more years with no problems after I replaced the Power supply. I do stay up to date with spyware and viruses. I don't get any messages, errors, blue screens or warnings when this happens. It's like there was a power outage but just to the PC. The machine came with a 80GB Western Digital Hard Drive and I added a 2nd 80GB Maxtor HD. I am in the market for a new Hard Drive anway because I am getting low on space. Not danderously low though, maybe 8GB available of the total 160GB. I will look into replacing the CMOS battery and inspecting all cables as well..thanks again. One last thing...the last thing I did was take everything apart and put it all back together one by one (Ram, Processor, Hard Drives, etc) Basically I unbuilt it and then rebuilt it. I didn't clean the heatsink and processor and apply new Thermal compound.....do you think that would be an issue? Remember this was happening before I took the whole thing apart too.

you should use thermal compound.

But even then, we shouldn't jump to the conclusion that your probvlem is heat related. Easiest way to check this out would be to run a temperature sensing program, or maybe check the temp from the BIOS, that works too.

If it is within normal operational limits, then start looking for other possibilities.

Ok, I downloaded 2 software sensors last night.
MemTest86+ and MotherBoard Monitor.

MotherBoard Monitor gave me a temp on the processor of 0 degrees C but kept coming up that my fan speed was too slow (falling below 1500 RPM)

MemTest86+ was giving me temps of my two internal Hard Drive and they were coming up at 39 degrees c and 45-49 degrees C, so I kept getting warnings on the Hard Drives.

Is there a way to remedy this or do I just have to get a new Hard Drive?

Flashing Green - System is in a low power state, either Standby or Hibernate.

Change the Bios Setting for Power
Also Windows setting for power managment and Hibernate.

if that dose not work strip the system down and try and rule out what part is casing it to do this.

Yes, front panel flashing green and won't let me power backup.
Not for a little while anyway.

I checked all power management options in windows and disabled hibernation, standby, and Hard drive shutdown.

I will check the Bios setting for Power....what should it be set to?

And should I be concerned about any of the system or Hard drive temperatures I mentioned above?

Thanks

I had a like what you are discribing but more random in the begining. After a year of looking and replacing parts it ended up being the mother board. On really close inspection I had leaking capaciters 8 total. You have to look close in bright light. They were all leaking at the bottom of the "can". I hope this helps. good luck and let me know

leaking capacitors? The board has given up the ghost.... time for a replacement.