@Feryll - This is a guess, but you probably have a shorted wire in your light fixture, or you could be using the wrong wattage bulb as this article talks about. Either way, you need to get an electrician to come out and take care of the problem before you have a real problem.
If the breaker is tripping then that's a big warning sign. You would be surprised at the shoddy electrical work in some homes, especially older homes that were built before codes were as strict as they are nowadays. Some people also doing their own wiring when they have no real understanding of what they are doing. This is very dangerous.