Simple and Microsoft licensing doesn't happen. First, you need to talk to your Microsoft or reseller licensing specialist, as the best way to license Microsoft stuff varies greatly according to your specific circumstances.
That said, basically, there are two entitlements that you need from Microsoft; the right to use Windows on a client device and the right to use Windows in a VM environment.
Your right to use Windows clients typically comes from one of 3 sources:
- The OEM or SystemBuilder license embedded with your PC or thin client device (ie. the sticker on your PC)
- A Windows 7 Professional/Ultimate retail box (NOT home versions)
- A Windows "Virtual Enterprise Centralized Desktop" license.
If you have Windows XP/Vista/7 on Software Assurance via your Select or Enterprise Agreement, that does not entitle you to run VDI XP/Vista/7 instances. Your volume license XP/Vista/7 permits you upgrade to the next version.
To legally run Windows 7 in a VDI environment, you must either:
Own (own as in your organization) a desktop or thin client with the right to use Windows; have Windows on Software Assurance, and purchase the "Virtual Enterprise Centralized Desktop for Software Assurance" license from Microsoft. (This is like $25/client MSRP)
Or if you are connecting from a non-Windows device, non-windows thin client or a third-party/consultant PC, you must purchase a full "Virtual Enterprise Centralized Desktop" license, which costs about $110 MSRP. Note this is per-device that connects to your system... if a business partner may connect from his laptop & desktop, you need two licenses.
Yes, it's confusing, expensive and complicated. If you're doing this in quantity, be very, very careful to fully understand what you are doing. I know of an organization that literally has a room full of XP boxes that they had to buy at the last minute to meet their licensing obligations. Microsoft is also making changes, so YMMV, and again, talk to and negotiate with whomever you generally buy Microsoft licensing from.
As always, get any and all commitments or promises in writing, as this area is a moving target that is impossible have complete understanding of. Make sure its well documented so your successors won't be screwed in the future.