If you work for a company that has a license agreement with Microsoft, any Microsoft software your company uses, Microsoft will allow the employees to use - You get access to their VLK.
Say you work for ABC company who uses Vista and Office 2k7. If your IT department wants to deal with all the annoying end-users, they can hand out Vista and Office 2k7 to everyone to use.
Now, if you hand it out and pass it around, the company isn't responsible. Microsoft may request user information or people who have left that may be using the software and then they can go after you.
You're entitled to use it for free as long as you work for said company and they are using it.