Hardware

It is possible that you could have fried something out on the motherbaord, however, it is also possible that you could have a bigger underlying problem. I had this happen to one of my older systems just a few days ago and found that it was the motherboard (many of the "caps" on the board were bubbled and leaking). Another thing could be that you have a virus (though I haven't heard of one that only does this when the keyboard is plugged in and not when it is unplugged). An easy test for your keyboard problem would be to take the keyboards to a different PC and plug them in (test them) to see if that PC does the same thing, but the problem with this is that if one of those keyboards have a short in them, you risk the chance of harming the other pc that you are plugging them into, so make sure that it is one system that you don't care about/need. Option 2 is to go out a purchase a new keyboard (you can usually find them at a local shop for ~$10-$20).

After getting feeds here, i opened up the keyboard and took the 'chip' out and found some gel like substance dried up on some of the circuits. i cleaned it up, plugged the keyboard in and viola!, the keyboard is working fine now!! thank you guys; i'm typing on my keyboard after such a long time and its all due to your help. i am really greatful. thanks :-)