Have you ever had your A/C pan overflow or ever noticed that rusty colored water coming from a pipe over your window?  If so, your primary drain line for your A/C is clogged.  An easy way to help prevent this from happening is to pour a solution of 1/2 a cup of water and 1/2 a cup of bleach down the line.  Do this 2-3 times per year and your line should operate as intended and prevent those unsightly stains or damage that may occur.