No POST on previously working system

About four years ago I built this system:

Biostar TForce4 AM2 motherboard 
AMD Athlon 64 3800+ @ 2.2GHz 
2x512MB Corsair CM2X512A-6400 DDR2 
ATI Radeon X1800XT (512MB RAM with dual outputs) 
Antec SP-500 Power supply 
Seagate 80Gig SATA 

It worked beautifully for about two years. One day it suddenly would not power on. It's exhibiting symptoms of a short (when you press power, comes on for 1/2 second) but I can't find a short. I removed everything but graphics, RAM and CPU, and no go. I even took the board out of the case and it did the same thing...could this symptom be a result of failed RAM? I'm thinking if I just replace the RAM it might start...Should I try new RAM or something else? I don't want to pay $70 for a service call if possible.

Thanks

Edit - 10/28/2010

Ok it was my power supply. I learned that a "dead" PSU can show the same symptoms as a short. Too bad I didn't figure that out for so long--it actually stopped working two years ago (within warranty period). Just didn't have time to dig in to the problem until now. Anyway, I am going with a Corsair this time which has a 5 year warranty. We'll see...

Solution 1:

When it doesn't beep, try for 2 times to power the machine on without cable attached, this is a PSU reset. If it still doesn't work, try another PSU (in 95% of cases you are now done). Then remove anything from the mainboard besides power, try switching it on. If it beeps, add the components one after another (CPU, RAM, rest), until it doesn't anymore (the last is your culprit). If it didn't beep without any components, its the board.

Solution 2:

Disassemble everything, give it a good blow (there's probably plenty of dust), and assemble again.

I had similar problem with dust in graphics card slot.

Also make sure that all cards and memory are pushed all the way down in their slots. A slightly 'raised' card can give similar symptoms.

Solution 3:

First off, are there any post beeps? If you don't have your internal speaker attached, do so and listen for post beeps, that will be your best indicator of what is wrong (you can find what each beep code indicates in your motherboard's documentation). When you try powering on with just the basics, make sure it is only one dimm of ram. Keep swapping out the dimms and try each one in a different slot. If you still have no success, you need to remove variables. Try with a different graphics card, different ram and a different power supply. If you still have no success, it is your motherboard (which based on the symptoms you describe, I'm leaning toward motherboard or power supply).