It could be caused by a few different problems.
First, check your nut. If your nut is worn out the strings can be rubbing against the lower frets as you play them.
Check your action. If it's really low, that can cause a buzz.
Check your neck. If it's too tight, you might need to loosen your truss rod a bit. You can look around the internet to find some good sources that will tell you how to check and adjust your neck. Just remember that when you do adjust the neck, ALWAYS use small turns of the truss rod, usually 1/8 of a turn. And always re-check after every adjustment.
I've had the same problem many times over the years, and a quick neck adjustment has always done the trick. Check your nut and action first, and if those don't seem to be a problem, go to this site: http://www.athensmusician.net/archive/2001-05-01_geneimbody1.shtml
It gives some pretty good info and shows you how to adjust your neck. You'll probably end up loosening your truss (giving a counter-clockwise turn), and that should solve all your problems.