Hello Kamalaksh,
If you visit the Microsoft Store, they will trade an Office 2007 product key for your credit card number and authorization ;-)
http://store.microsoft.com/microsoft/office/category/202
Buy Office 2007 Now, Get Office 2010 Free http://office2010.microsoft.com/en-us/tech-guarantee/microsoft-office-2010-technology-guarantee-FX101825695.aspx?CTT=97