Windows OS license is definitely per instance - at best that's per machine, but usually it's per VM on each machine. If the OS was acquired from an OEM vendor then it's usually locked to that piece of hardware and can't be ported to another computer if the first dies. This often isn't enforced in the license key itself, but is in the agreements.
Typically Microsoft applications (developer tools, office, etc) are per user - meaning as long as the person is the primary user of the computer (typically 70% or more) then the product can be installed on multiple computers. There may be a limit of three or something. I've been through a few audits and never seen them blink as long as it was clear they were assigned computers to an individual.
That said, if you enter a program like BizSpark then you can generally get all of the licenses you need for any computer owned or operated by the BizSpark entity as well as employees.
I am not a lawyer (IANAL), but I believe you should review the end-user license agreement (EULA) for each piece of software to determine if this is allowed. Google 'Microsoft Windows EULA' if you don't have your Windows installation disc/box on hand.
While I have certainly seen software applications that allow what you are looking to achieve (single user (at a time) with multiple installations), it is rare (in my experience). For MS Windows (and pretty much all other Microsoft software), I'm pretty sure it's not allowed (but, again, don't take my word for it... check the license).