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Alphaa10, the use of a virtual machine (VM) versus a physical PC is irrelevant to the OS license issue.  You can’t use the same Windows product key on two PCs – whether virtual or physical, to the best of my knowledge.  I would be surprised if the Windows Genuine Advantage security tool didn’t detect the duplicate license in use.

Again, I encourage you to simply pick up the phone and call Microsoft to find out what can and cannot be properly done.

That’s a nice summary, Brian.

I do agree that there has been much published ‘confusion’ about using a single Windows license multiple times in a multi-boot manner on one PC, and I don’t know the “official” answer on this question.

However, unless I am misunderstanding, Alphaa10 appears to be installing an image of one of his customer’s PC on his own PC for support purposes.  That isn’t a multi-boot situation – it is two instances of one Windows license on two separate PCs.  It seem a questionable practice to me, but again:  I have no ability to offer a legal opinion, just the advice that Alphaa10 might wish to contact Microsoft directly to pursue the issue.

By the way, in respect of this viewpoint about virtualization:
alphaa10 wrote on Apr 15^th, 2008 at 1:42pm:

Leaving aside how far this is off the mark with respect to licensing, it's completely the wrong way to understand VM performance.

Actually, the only noticeable overhead they have is in emulating I/O, because the normal user code in the VM is almost completely running natively on the CPU. The bulk of what virtualization penalty you experience comes not from the virtualization process itself as such, but rather it comes from having the virtual environment running on top of a full-blown desktop OS leading to a trip through the host OS's filesystem.

If you eliminate the host OS (run, say, VMWare ESX or something similar which is just a thin bare-metal hypervisor) virtualization imposes virtually no overhead, and the benefits remain immense. So immense, in fact, that the surprising effect of virtualization is that at a large enough scale, things can get faster.

The big thing that virtualization does by allowing things that would previously run as separate machines or be on separate disks to all run shared on the same hardware is lift overall resource utilization immensely.

Normally, machines have to be provisioned to deal with peak loads, rather than their averages. As long as the loads on different machines are not closely correlated, by using virtualization to get them consolidated on shared hardware you can use the cost savings on other things, such as 15K RPM disks or dual redundant power supples, and get the benefits of those things on all the VMs.

When you combine that with the ability to move VM images around to hardware and provision them on the fly based on load - which is what you can see in action with Amazon EC2, for instance, which will even replicate the things around the world for you - the results of doing things to increase the average load factor of machines up higher is big savings overall that you can invest instead in doing things that benefit all the VMs which wouldn't be cost-effective if the machines were physically separate.

To build upon the prior comments of Nigel Bree, one of the performance characteristics of VMware Workstation is its ability to share memory between the host and guest:

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In addition, a VM can be configured to access a physical disk on the host system; or a virtual disk, the storage of which can be either pre-allocated, or can grow dynamically as needed.  These three options disk provide, in order, the best-to-good range of performance.

Alphaa10, please report back on what you learn from your conversations with Microsoft regarding the licensing issues.

Pleonasm said, "However, unless I am misunderstanding, Alphaa10 appears to be installing an image of one of his customer’s PC on his own PC for support purposes.  That isn’t a multi-boot situation – it is two instances of one Windows license on two separate PCs."
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Now I understand where some of the confusion in this discussion comes from-- I do not run my clients' images on my own system. Aside from understanble privacy concerns from the customers, my only objective is to replicate the problem on a healthy OS.  To this end, I merely duplicate the behavior of the client who met a problem.

Any licensing issue that might arise comes from running multiple copies of the same OS product key. As one poster observes, the old Borland argument that multiple installation to, say, a laptop and an office desktop, does not violate the license so long as both are not used simultanously, no longer applies with a VM.

The whole licensing landscape is in a state of "aftershock" from the invasion of VMs. None of the companies wants to appear greedy, but appearances aside, most are hounded by the thought of how much loot they might extract by a different licensing regime. Just as MS "discovered" that it could keep its mighty river of revenue from Windows licenses flowing by moving to a subscription model, the VM probably will bring something on the same order, no longer based on a count of machines. 

But what is "fair"? MS can speak for its own interests, but these, too, rest on customers who buy their licenses. Without paying customers-- ie. customers rejecting MS terms and moving elsewhere-- MS attorneys will be forced to find a new career. And MS might be forced, once again terrified at the prospect of losing its revenue stream, into being nicer to the customers who made it obscenely weallthy.