Windows – Same Windows 7 license key on a laptop and a desktop — legal

According to official Microsoft document from http://download.microsoft.com/download/5/f/4/5f4c83d3-833e-4f11-8cbd-699b0c164182/royaltyoemreferencesheet.pdf

The OEM vesions of Windows Vista Business and Windows Vista Ultimate include downgrade rights to Microsoft Windows XP Professional, Microsoft Windows XP Professional x64 Edition, and Microsoft Windows XP Tablet PC Edition. Customers may not downgrade to Windows 2000 Professional from Windows Vista Business or Windows Vista Ultimate. No downgrade rights are included in other Windows Vista products in the OEM channel

Translation: Legally you can downgrade OEM versions Vista Business and Vista Ultimate. Other version cannot be legally downgraded.

Quoted from this thread Windows 7 licensing rights:

What license do you have? Retail, OEM, and Volume Licensing are all different.

Volume Licensing http://windowsteamblog.com/blogs/springboard/archive/2009/08/06/windows-7-enterprise-edition-understanding-software-assurance-volume-licensing.aspx:

"Virtual OS Rights - Use up to four instances of Windows in virtual OS environments for each license that has active Software Assurance coverage." Notice that this appears under MDOP. "MDOP is an add-on subscription license"

You can find the retail and OEM EULAs here: http://www.microsoft.com/about/legal/useterms/default.aspx

As far as I know, since the days of Vista, Microsoft allowed running up to 4 virtual machines, as long as they use the same operating system and serial number as in the physical machine.

EDIT

It's finally quite unclear what are the virtualization rights of a desktop Windows version, with conflicting opinions between authorities (even Microsoft). Finally, only a thorough examination of the EULA can maybe give some answer (and maybe not).