Homeowners have plenty of reasons to replace their windows. They may have deteriorated to the point that they're ugly or drafty. They may be difficult to open or to keep open. They may be hard to clean.

You may want to minimize your home's energy use and impact on the environment. You may also want to install a window where there was none before.

But buying new windows is a very expensive home improvement. Although window sellers make extravagant claims about how much their windows will reduce your energy bills, even new windows that replace old, very drafty ones won't "pay for themselves" in energy savings.

If you're thinking about buying new windows, know that choosing a good installer is as important as choosing the right windows. Unfortunately, in addition to sketchy claims about the environment, some companies abuse customers with high-pressure sales tactics and substandard products and installations.

The nonprofit consumer group Delaware Valley Consumers' Checkbook surveyed its members and Consumer Reports subscribers about their experiences with area window-replacement outfits and received a disturbing number of complaints about sloppy installation work. Until Nov. 11, Checkbook is offering free access to its ratings of window suppliers and installers at www.checkbook.org/inquirer/windows.

Companies differ greatly in the quality of work they do and also in the prices they charge. Checkbook's undercover shoppers collected price quotes from area contractors for several typical installation jobs and found big price differences. For example, for one carefully specified replacement job, its shoppers received quotes from area companies that ranged from $1,750 to $3,620.

Start by comparing several window types and models. Ask about and assess how each option will affect your home's appearance, the amount of light admitted, your comfort, energy savings, and how long the windows last. For many homeowners, costs will drive decisions. You'll find a very wide price range for each type of frame material, but in general, vinyl is the least expensive option; wood is usually mid-range, with exterior-clad wood more expensive; and fiberglass models top the price list.

Check salespeople's claims about energy savings with information available from the independent Efficient Windows Collaborative (www.efficientwindows.org). For details on durability, check the results of Consumer Reports' tests. Also compare warranties: Better-sealed window units tend to come with warranties of 20 years or more and don't prorate reductions in the covered value as time passes.

Once you've decided on a window model, obtain several written price quotes. You'll find enormous price differences for the same windows and work.

Whichever company you hire, make sure you get an airtight contract. It should include:

• Product details and installation procedure. Know whether flashing will be installed, whether the windows or trim will be painted, the size of window openings and glass area after installation.

• Insurance information. The contractor should carry two types — general liability and workers' compensation — and be willing to show you a certificate that confirms coverage.

• Payment schedule. You should be able to pay all, or at least half, the contract price after the work is complete. The more you leave to the end, the more leverage you'll have to make sure the work is done satisfactorily.

• Work schedule. The starting date should be firm, so you can prepare for the job. A completion date is less important because most projects can start in a week or less and take only a day to complete. But it's wise to add a phrase indicating that the work will be continuous and a note about who will be on site supervising the job.

• Quality assurances. To provide some recourse if the job proves to be obviously substandard, contracts should contain a phrase to the effect that the contractor will complete the project in a workmanlike and professional manner.

• A cleanup plan. Because window-replacement projects generate a lot of construction debris, carting it away (and paying disposal fees) should be part of the contract.

• Compliance with lead-abatement laws. If you have any reason to believe your home contains lead-based paint — and if it was built before 1978, it probably does — ask contractors to show proof of their lead-renovator certifications. In any contracts you sign, include a statement requiring contractors to follow EPA regulations for containing the work area and minimizing the generation of lead-paint dust.

Delaware Valley Consumers' Checkbook magazine (Checkbook.org) is a nonprofit organization with a mission to help consumers get the best service and lowest prices. It is supported by consumers and takes no money from the service providers it evaluates. Access Checkbook's ratings of area window suppliers and installers free of charge until Nov. 11 at www.checkbook.org/inquirer/windows.