b2run now would be the time to make the upgrades, especially with the government tax credits you can get right now. Don't know how long those will be around, take advantage of them while you can.
I do know that replacement windows are covered under IRS Form 5695. This is a link to a search for that form on the IRS website.
IRS Form 5695
Look at the 3RD entry in the search page it is the 2009 form of 5695 (still in draft form right now) The 2nd entry is the details for this for form, it reads Field Reference TY2009.
I am fairly sure that any EnergyStar approved product that you will be eligible for the tax credits. But do not take my word on this(do the research). This is a link to EnergyStar's page for Residential doors, windows and skylights.
Doors&Windows.
PLEASE READ THIS:
I am not a tax or construction professional. Before starting any home improvement/upgrade do a little research, gather your information then consult YOUR TAX PROFESSIONAL.This way you are not disappointed come tax time. Form 5695 covers Solar Electric, Solar Water Heaters, Wind Energy and Geothermal Heat Pump installations. This form will also be used for qualified high-efficiency items like heating and cooling systems, doors, windows and insulation.