Re: PC won't power up w/LAN plugged in. [SOLVED]
Old post, but I just had a similar problem, so thought I'd share my finding in case someone else stumbles on this post. In my case, same symptoms - was a working (older) PC, then one day it wouldn't power on. By somewhat random troubleshooting, I figured out that if the ethernet cable was unplugged, it would power up and appear to work fine, but with ethernet cable attached, it wouldn't power up. Googling the symptom got me to this original post (and several others) but none of them had a solution.
I finally figured out it was the Standby 5v output of the PSU that was bad. I put a meter on that pin of the ATX PSU connector (typically the purple wire), and it read 5v as expected, but when I hit the power switch, it dipped to ~1v. (I'd also get a blink on the keyboard lights, but the PSU wouldn't "start.") Then I unplugged the ethernet cable and repeated the test. The standby lead dipped to ~4v, but the PC started. Apparently the NIC card draws enough current from the standby 5v line to pull that rail down only when the ethernet cable is connected. The NIC card using standby juice makes sense as it needs to be powered for WOL to function.
A related observation is that in this faulty condition of the PSU, the sleep function of the PC stopped working as well. The PC would appear to sleep properly, but when trying to awaken it, the PC would always cold-boot, which is akin to an "unexpected shutdown"/crash. This also makes sense as the PC needs standby 5v to keep the memory alive during sleep. On this motherboard, there is a small LED next to the memory sticks that indicated the memory is powered. That light was going out during sleep, indicating it wasn't being kept alive.
A bit more troubleshooting, I attached my bench supply (set to 5v) to the purple lead of the PSU as a "helper," and the PC worked properly. Started right up, sleep function worked (and memory LED stayed lit), awoken by WOL, keyboard, etc. Clearly I found the problem!
I pulled the cover off the PSU expecting to find the typical bulging/leaking capacitor, but alas, everything looked normal, so no quick fix there. Fortunately I had a spare PSU that I was able to swap into this PC and it's working fine again. The bad PSU is back in my [way too big] "to be repaired" pile, which may or may not ever get my attention again.
Here's hoping someone else stumbles upon this posting and finds it useful to this rather perplexing problem.