Dell Optiplex 780 freezes on Windows 7

Hello,

I've been experiencing a very frustrating problem on my PC. Around 2 minutes after turning it on and entering the OS the screen freezes and there's nothing to do but force the PC to turn off pushing the power button.

I've updated the drivers, checked all the conections and nothing. It also seems that it happens more often whenever I connect the Ethernet cable before turning it (the PC) on. I don't know if that has anything to do with the problem but I need to fix it ASAP since it is the computer I work with.

Here's a couple of screenshots of the PC characteristics (I hope you don't mind it is in Spanish):