You will need to determine the manufacture of the window. If the contractor that installed them is still in business he should be able to help. If the "dirt" is in between the glass, most likely the insulated glass seal has broken, this is whats also known as a stress crack. It is very common with windows that get direct sunlight. Depending on the window, you may be able to replace just the glass, however on some newer products you will need to replace the entire sash. If the contractor is no longer in business I can try to help, just give me a call at [protected]
As a contractor and a previous window manufactures representative, what they are telling you is true. If the moisture was between the glass than you would have a valid argument, however since the condensation is on the inside of the home, it is condensation trying to leave the home and cannot get out due to the windows having a tight install. I strongly recommend that you get a dehumidifier and let it run for a while. This problem should disappear.
