If you purchase or have purchased Vista Home Premium, Vista Business or Vista Ultimate betwee 26 June 2009 and 31 January 2010, you are eligible for an upgrade to Windows 7.
The main website is – https://windows7upgradeoption.com/Landing.aspx
All the FAQs on this can be found here – https://windows7upgradeoption.com/Faq.aspx
Furthermore, if you have purchased a branded computer from one of the following vendors, you can get the upgrade offer directly from the manufacturer.
Dell – https://win7.dell.com/Eligibility.aspx (only up to 31 Dec 2009)