|It's the capacitors on the MOBO. Open the case again, and take a look. If you can't see them all, lift up the protective plastic cover for the case fan. You'll see that they are popped up. That's the cause of your Thermal Event and the inability of the machine to boot.|
If the desktop is still under warranty, you can get a replacement MOBO from Dell. Dell's out of warranty replacement for these boards ran out on 31 Jan, so, if you're machine is out of warranty, you'll either need to buy a board (appx $500), or buy another computer.
Life's more painless for the brainless.