Good windows installed badly won't deliver the look, comfort, or savings you expect. So in addition to getting multiple bids and detailed contracts, here are some specific tips on choosing a window pro:
Many major window manufacturers train and certify installers on their specific lines. Using the same contractor for purchase and installation can avoid finger-pointing if there is a problem later. Also look for certification from the American Window & Door Institute at www.awdi.com or InstallationMasters at www.installationmastersusa.com.
Go online and read the installing replacement windows instructions for the window you've chosen. Ask whether the installer plans to follow them—down to details such as the type, amount, and placement of flashing and insulation. If not, he should be able to explain why. Remember that some changes while installing replacement windows could void the warranty.
Any seasoned installer should be able to tell you how long the actual task of installing replacement windows should take. Ask about the installation process. It's better if each new window is put in immediately after the old one is removed. If one crew removes all the windows first and another installs, you'll be left with multiple openings for the duration. And be sure the installer measures each replacement window before he rips out the old one. Otherwise, you could end up with a boarded-up hole waiting for the right window to arrive.
If you want to paint after installing replacement windows, have the installer use acrylic-latex caulk, which is paintable, not silicone. And be sure the window works well before the installer applies interior trim.