I have a Dell Dimension E521 that was purchased new in 2007. I bought it with 1GB of RAM and immediately upgraded it with four Crucial 1GB DDR2 667 (CT12864AA667). A year later, I replaced the hard drive so I could load Windows 7 Pro without risking any data.
The only other change we made was the addition of a Zotac PCI Express x16 video card about two years ago (to add DVI functionality).
About 2 months ago, the system began freezing (no mouse or keyboard actions recognized) and then shutting down or restarting randomly. There are no signs of the action in the Event Log.
I have performed Dell Diagnostics and all components passed. I used my Antec power supply tester and it passed the test. I swapped to another hard drive and loaded a fresh OS and the issue followed it. I disabled the onboard NIC and installed a PCI NIC with no improvement. I installed Core Temp 1.0 and the CPU cores are running between 29 and 35 degrees Celsius. Case and Power supply fans are running.
I see no obvious bulging capacitors or overheated visible components on the motherboard.
In short, I ran through a bunch of diagnostic tricks and have found nothing. Sometimes just talking it out reveals something, so does anyone have any additional suggestions?