For EmployeesSome organizations purchase a volume license for Windows that allows them to install and activate an unlimited number of installations of Windows. The enterprise agreement and organizational policy may state that employees can use the Windows installation for a home computer (or a computer at home used for work purposes). To find out if this is possible, ask your IT department.
Some schools purchase a volume license for Windows that allows them to install and activate an unlimited number of installations of Windows. The enterprise agreement and organizational policy may allow them to install or upgrade to Windows 7 on your personal computer that is used for class work.
An alternative to using the desktop operating system is to use Windows Server 2008 R2, but convert the installation to include many of the desktop features such as Aero. Windows Server 2008 R2 is freely available to students from Microsoft Dreamspark.
For Business OwnersAlthough this isn't free, it may be possible to license Windows 7 for your organization and take a tax deduction on your business' tax return. Seek the advice of an accountant that is familiar with your country's tax laws and practices to see if this is a possibility.
Other MethodsUnless Microsoft offers another program to provide Windows for free, the only other methods to successfully activate Windows without buying it are illegal. Most of these involve installing a crack or cracked version of Windows or disabling the Activation component in Windows. These types of activities are not recommended because it is possible for cracks/cracked versions to contain viruses and other malware. Additionally, some of the changes that may be required could render the system unstable and unusable. Microsoft periodically releases updates that undo most of the cracks that are available, so the illegal version may stop functioning after an update is released. It may also be possible to find keys online through warez sites and other parts of the Internet's underground, but Microsoft periodically invalidates installations that use these keys. Besides the technical difficulties presented, it is also possible to be prosecuted under criminal law and to be sued by Microsoft in a civil trial in the event that you are caught. The moral of the story... If you need Windows, buy it!
AlternativesIf you need a permanent operating system, consider a freely available operating system such as Linux. Some applications can successfully run under an emulation layer such as wine (Windows emulation). Other applications have direct ports to Linux or can be recompiled to run under Linux. Some open source alternatives also exist for commonly used Microsoft software/formats. One example of this is OpenOffice (compare with Microsoft Office).
If you need to run Windows temporarily, get the trial version as it will allow a number of days of testing before it requires activation (I believe that 60 is the current number...).