Not at the same time. You can install it on as many as you want ONLY upon contacting Microsoft and having them reset the key. This is because the key is locked to the motherboard. This is different from the OEM version which can NOT be reset even by contacting Microsoft.
Let me clarify. The OEM version is permanently locked to the motherboard or pc it was installed on and cannot ever be installed on another computer. The normal and more expensive version (the one I purchased) CAN be installed only on ONE computer at a time. In order to install it on a second, or third, or etc. it wil have to be removed from te previous computers and you will then have to call Microsoft and tell them you are going to install it on a new PC. They will at that time RESET you key so it can be installed on another PC.
As many as you want, as long as you only have it installed on one at any given time. The key is only good for one computer at a time, but you can have the ability to install it on another computer after you contact Microsoft and have them "reset" the key(if you continue using the previous computer it will from then on show up as not a genuine product anymore, no longer able to update or be supported), then you can safely install it and activate it on the new computer system. Also, any "computer" I have read is defined by Microsoft as any device(such as hard drive) to which the Windows software is installed.
The OEM version is much less useful. It can only be installed on ONE computer...ever. Now, you can re-install it on the same computer over and over as many times as you want, but only that one.
No. The license windows provides is for use on one computer at a time. Otherwise Genuine Advantage will not activate and you will not be able to receive certain windows updates and some functionality of the program may be limited.