The best time to replace windows is when you replace siding.   Windows have improved a great deal over the years.  Getting the right windows and installing them correctly to seal your home against water intrusion is critical.  New windows save you money in electricity and reduce hot and cold spots in your home.

Windows we typically use are: Cascade, ALSIDE and PlyGem

http://cascadewindows.com/index.php

http://www.alside.com/products/windows-patio-doors/

http://www.plygem.com/wps/portal/home/brands/plygem-windows/gallery