One machine at a time per License Key.
If you uninstall it (ie: Wipe a computer on which it is installed) you can reinstall it on another computer.
Installing it multiple times on a single machine may cause a problem as there are AA (Academic Alliance) and RTL (Retail) Keys. Academic ones are supposed to activate once, Retail activation can be used may times.
I have used keys from MSDNAA multiple times however, so I suspect RTL Keys are sometimes distributed, and either way they are both covered by the same License.
If activation fails for a subsequent install or you want to install it on a different computer after you have uninstalled it on your current one, make a quick call to MS and they should give you a code to complete activation.
MSDN Subscription Software Use Rights contains…
Need the full details? Software use rights for customers who have purchased the license through a Microsoft Volume Licensing program are governed by the Product Use Rights Document . All other MSDN Subscribers are governed by the Retail Software License Terms select “MSDN Subscriptions” for the Product Name).
Which leads you to… Microsoft Product Use Rights, Product List, and Licensing Topics …excerpt below:
Desktop Operating Systems – per copy, per device license
You must acquire a license for each device on or from which you access or use the software (locally and remotely). You may install only one copy on the device. You may install that copy on the host operating system or in a virtual (or otherwise emulated) hardware system. In Volume Licensing, the desktop operating system license is an “upgrade license.” You may only acquire upgrade licenses for devices for which you have already licensed a “qualifying operating system.” A list of “qualifying operating systems” that qualify for an upgrade license is contained in the Product List, which can be found at Microsoft Product List for Volume Licensing.
If you acquire “Software Assurance”, you have the right to use Windows Vista Enterprise Edition on the device instead of Windows Vista Business. This also permits you to run up to four additional copies or instances on the device.