How Many Windows
Consider which windows you want to replace. Do you want to replace them all or just certain ones?

Budget
Prepare a preliminary budget. How much you can afford to spend will determine how many windows you can actually replace based on window materials, features and designs.

Features
Consider what you want your new windows to do. Are they to be an architectural design feature or merely provide functionality? Decide whether you would like the windows to open and close, and how they should operate. Don't forget that windows also provide security.

Energy Efficiency
Discuss the energy-efficiency options (such as standard insulated glass and low-e glass) offered by the vendors. Consider low-e window coating (a window coating that reduces heat loss through the glass while at the same time allowing heat from the sun to penetrate) or gas-filled windows (argon or krypton inserted between double glazing which reduces heat loss). Work the additional cost of these features into the context of the estimated savings on your heating and cooling bills. Do the expected savings justify the extra cost? Aluminum and vinyl window frames may be less expensive than wood-clad vinyl or aluminum, but they won't provide the same insulating capabilities.

Quotes
Contact several replacement-window vendors and installers for quotes. Make sure to receive a written no-obligation estimate.

Check References
Ask the vendor/installers for references and check them: Were past customers happy with the work done? Was the work completed in a timely manner? Have they had any problems since the installation? Did the contractor leave the site in a clean and undamaged condition? Check with the Better Business Bureau to see if there have been any complaints lodged against them.

Guarantee
Make sure that the vendor/installer offers guarantee(s). Sign a contract for the work. If ordering the windows separately from a vendor, make sure that the installer has signed off on the list to verify that the windows specified are correct.

Insured
Make sure that the installer is properly insured. Get a valid certificate of insurance from the contractor before you pay any money or before the work begins.