How are you going to get new clients? There are many possibilities. One approach is get free or large price reductions for the initial window cleaning to get testimonials. Once you get credibility, you can charge your regular price. How are you going to market it besides word of mouth? It can be flyers / brochures, website or placing ads in local media, or face to face with your potential customers.
Personally, I would use all of those methods and think of some gimmick to get free publicity, such as doing free window cleaning for non-profits or some local citizens or businesses that have encountered bad luck. Or if you really wanted to go for it, pick one day as a free window cleaning day. Sonic, Ihop, and Denny have gotten plenty of free publicity for their freebies this year.
I just think with a tough economy, you need to think out of the box to be successful.