I've found yell.com to be a complete waste of time in the past, but that wasn't running a window cleaning business so it might be different for window cleaning. However, think about how you look for things - do you go to Yell.com or do you just go to google? I know what my answer is, and suspect for most people it is just a case of googling it. In that case I would advise spending the money on SEO instead