When shooting indoors with my

When shooting indoors with my 300d (base rebel digital),I have been encountering some light metering problems.I use the 430ex as an external flash. While shooting a subject in front of an external light source-(window, etc.), no matter how small of the frame area include the outside light source, the cameras light meter will invaribly expose the entire image based on the outside ambient light, causing the image to be grossly underexposed. Do you have an explanation/solution for this aggrivating problem?