In this article, reporter Scott Dunn noted that the Service Pack 1 version of Windows Vista gives users the option of buying the 'upgrade edition' and installing it on any PC, which enables them to avoid paying for the more expensive 'full' edition.
Well to give you a perspective in U.S., the list price of the upgrade edition is more than $100 cheaper than the full edition. According to Dunn, the same option was available to users when Vista was first released, and Microsoft's failure to close this loophole in Vista SP1 suggests that the vendor "approved the back door as a way to make the price of Vista more appealing to sophisticated buyers."
But interestingly a Microsoft spokesperson disputed the notion that the vendor supports users taking advantage of the technical loophole in Vista by commenting as "Just because a piece of software installs on a PC, doesn't mean that it is properly licensed. The licensing states that upgrades require a fully licensed version of Windows to be eligible to use an Upgrade license."
News source: CRN