Microsoft has introduced the technological guarantee, stating that if you have recently purchased a license of Office 2007, you are eligible for a free upgrade to Office 2010. Depending on which version of Office 2007 you purchased, you may upgrade to the 2010 counterpart, or possibly a higher one (e.g. Small Business 2007 upgrades to Professional 2010, neat!).
To be eligible, you need to have purchased, installed and activated Office 2007 between March 5, 2010 and September 30, 2010.
When you qualify, surf to http://office.microsoft.com/en-gb/tech-guarantee/ and fill out the form. You can choose to get the electronic download, or order a DVD for an additional fee. The license upgrade itself is free; you receive your key via the website upon completion of the form, and via e-mail.