isamuelson is right correct if you want a legal working copy of it then you need to purchase it. If you are trying to get it for free you won't find that information here. The exception to this would be if you were included in the free upgrade program that was being done right before Windows 7 released. If you were you need to contact the manufacturer of your system for details, this info should also have been packaged with your system if applicable.
If you would like to try Windows 7 you can download a 90 day trial of Enterprise edition from Microsoft.