Double pane windows can fog up and get dirty in between the glass panes. This is a huge problem for many people because it is almost impossible to clean between the glass. In fact, if you call in a professional window installer, they will probably tell you that you are going to need new windows. This isn't the case. While it is a much more involved process than you might be hoping for, there is a way for you to defog double pane windows and actually clean them as well.
Drill a 1/8 inch hole in the top corner of your window pane. Do this carefully, and cool the bit with water while you are drilling. This should be done on the inside of your window as well.
Drill another hole, same size and same method, in the opposite bottom corner of the window. The idea behind these holes is to allow any of the fog or condensation that is between the panes of glass to escape. Allow the window to stay that way for a day or two, or until the fog is gone from between the glass panes. If the fog was your only problem, you can skip to step 6.
Take a section of the small plastic tubing and insert it into the bottom hole in the window pane. Make sure there is enough tubing inside the window to siphon liquid out of it, and surround the hose with putty at the hole in the window. Also put a coffee can or bucket at the other end of the hose to catch the liquid.
Use dental water pick or a funnel and more plastic tubing to spray the inside of the glass through the top hole with either alcohol or glass buffer. The glass buffer is much stronger, and will clean hard water stains better than alcohol, but alcohol is cheaper and easier to use.
Allow the bottom hose to siphon the liquid out of the window while you are cleaning. If this isn't working, you can attempt to attach the small tubing to a wet/dry vac. Let the window dry out for a couple of days after you are finished.
Cut a circular piece of clear packing tape, about ¼ inch in diameter to cover each hole. Then poke small holes in the tape using the needle. This will allow air and moisture to escape from inside the window panes, and will keep insects out.
Do not attempt this method if you have tempered glass, or if you aren't sure. Drilling into tempered glass will shatter your window.