Hi,
for technical requirements and eligibility - please see here - http://www.energystar.gov/index.cfm?c=tax_credits.tx_index
Normally - when you purchased windows - for eligible items - you should get a certificate that this purchase is eligible for deduction - keep this certificate for your record.
To claim a credit - use IRS Form 5695 ( instructions included)
Let me know if you need any help.
Tax Preparer
Taxes, Immigration, Labor Relations