From TechNet Subscriptions Faq....
We understand that TechNet subscribers have unique needs, so we have
included special activation privileges for TechNet subscribers.
TechNet Subscription product keys can be used to activate up to 10 machines.
Although you must go through the activation process each time you reinstall
on the same PC, your total number of activations will not be decremented.
For Windows, TechNet subscribers have a 60-day grace period before the
product must be activated. If you will be reinstalling the software in less
than 60 days, you do not need to activate it. Please note that Windows
Update and Windows Genuine Advantage require that the installation be
activated.
For Office 2003, TechNet subscribers have 50 launches before the product
must be activated.
Software may be reinstalled on the same machine as many times as needed
without reactivating it. However, if the hard disk is reformatted prior to
reinstallation, reactivation may be required however the total number of
activations will not be decremented.
Is Microsoft Product Activation the same for the Windows operating system as
it is for applications such as Office and Visio?
Customers can activate the software during initial setup. Windows Welcome
will guide the user through activation. Users who choose to skip activation
during setup will be reminded periodically during the following 60 days to
activate Windows (15 days for pre-release versions). Subscribers who choose
to activate Windows after setup will be guided through the activation
process.
With applications such as Office and Visio, activation can be completed
after initial setup. The user will be asked to activate when an application
is launched. The user can choose to activate at that time or may choose to
skip activation and do it later; users are provided 50 launches of the
product before they are required to activate.
TechNet Subscription product keys can be used to activate software on up to
10 computers. If you plan to reinstall before activation is required, it is
recommended that you not activate.
http://technet.microsoft.com/en-us/subscriptions/ms772427.aspx