Burnt smell from laptop.

Recently (about 1 week ago), i accidentally plugged the AC/DC plug into the serial port next to the hole that the AC/DC plug was supposed to go into on my laptop. That action of mine didnt last more than 2 seconds before i realised that i was trying to plug the AC/DC into the wrong slot. But what i didnt know was the socket power switch was on... and my hibernating laptop awoke but only for a few seconds before it shut down and a slight burning smell was detected with the strongest scent coming from the fan area. After that i tried to power up my laptop and all the LED lights came on but only for 3 seconds before going off. I tried powering up with and without the power adaptor and the same thing occured. THe led lights will power up for 3 seconds before going off. The laptop just couldnt be started up. My warranty have already ran out of its due date. So i sent it to a tekkie who opened up my laptop to salvage my HD. The hard disk survived and all data are retrievable. The tekkie told me he couldnt see anything that was burnt or charred on the motherboard but he reckoned it was the motherboard that was fried. I need a second opinon and i was wondering if any one of you out there can tell me what are the possible explanations for what had went wrong in my laptop. Its a dell 600m that i was previously using.
Any help will be greatly appreciated.