Summary:
Upgrade versus Full Editions
You can use an Upgrade version of Vista to install on a empty PC
Legal ? … not sure, but it is an official Microsoft technique
So potentially you could buy an upgrade version of Vista rather then the Full version.
Activation
After using Vista for 30 days you must Activate it or Vista will drop back into a reduced functionality mode
Microsoft has detailed an official ‘hack’ that allows you to delay activation for up to 120 days!
The following are written by Brian Livingston:
Please read his website at: http://windowssecrets.com/