Laptop switches off when I plug in AC adaptor

My laptop was running low on power this evening and when I plugged my AC adaptor into the socket the laptop switched off. I am able to switch it on OK just running on battery but as soon as I plug in the AC adaptor, it switches off. If I try to turn it back on with the AC adaptor plugged in, the power lights flicker as if it's trying to make a connection but it won't turn on. As soon as I remove the AC adaptor I can switch it on again. Problem is of course that now the battery is completely drained so I can't access the laptop at all!!
The on off switch on the laptop definitely working long as AC adaptor isn't plugged in. It's as if the AC adaptor is switching the laptop off because as soon as I plug it in you can actually hear it clicking off. My laptop is an Evesham Explorer 2007 which I have for over a year now and have upgraded the RAM to 2GB some time ago.
It usually runs very well and first time I've had any problems, All my coursework is on the laptop and I need to access it within the next week. I managed to save the most vital files to the family PC before the laptop ran out of power completely. Do I need a new AC adaptor? Help!!