While it is possible, i think you will run into problemswith hard drive serial numbers, etc and you will find most of the other computers you will not be able to activate the OS. Instead you need to look into microsofts open license agreements.
Microsoft Open License Charity Overview
The Microsoft Open License Charity program allows eligible nonprofit organizations to acquire multiple software licenses - rather than multiple software packages - at reduced prices. You need only purchase one complete software package and enough licenses to cover the remaining amount of computers.
Qualifying for Microsoft Open License Charity
To qualify for the Open License Charity program, charitable organizations must be qualified §501©(3) organizations as defined by the United States Internal Revenue Code. Organizations must demonstrate their tax status with documentation before they can open an Open Charity License agreement. http://www.microsoft...harity_faq.mspx