The small print in the contract
Windows and Office are licensed, not sold.
No one can buy Windows or Microsoft Office: instead users purchase a permission to use them. The license describes the terms of this permission. It is the restrictive legal text you have to click "OK" to upon install.
You must abandon many rights to use the software.
There are a number of restrictions that you must accept by law. Restrictions on who can use the software, what kind of revenue you may earn with it, on how you choose to install it, restrictions on your privacy, even on whether you can give it away: the list is long. Reading the license and enumerating your remaining rights is itself a difficult task.
An OEM-distributed software cannot be transferred to another computer.
If you bought your computer with Windows or Office pre-installed (the so-called OEM licenses, or the "shrink wrap" Windows discs), if you change computers you must buy software again. The license is linked to one computer, and expires when the computer dies. It is then illegal to transfer the software on another computer.
If you go to most of the free software sites you can click down a couple of levels and find the [GNU] GPL, the X license, the Apache license, whichever terms and conditions you have to accept in order to use that software.
Now with a proprietary software company, the license is buried so you can't read it until after you have paid for the product, then they're asking you to turn off part of your brain, they're asking you to turn off part of your ability to work with other people and to do business, when you use their software.
The meaning behind
Companies like Microsoft like to assimilate their software to physical products, when mentioning copyright infringement for example. Yet, proprietary software is very different because of the restrictive license -such restrictions would be unthinkable on a car or bicycle, for example.
Restrictions on the use of Office and Windows are so harsh, that many violations occur everyday around us. People are tempted to buy only one version of Microsoft Office and install it on two computers. Others keep their version of Windows when they throw their PC away. Other people give away their second-hand Windows software when they stop using it.
You have an alternative to breaking this law, or feeling very restrained when you abide to it. GNU/Linux is Free Software (much better than just freeware): its GPL license is designed to protect your rights.