Indeed, the type of person most likely to benefit from this workaround are power users and hobbyists who own multiple computers running Windows as well as Linux and Mac OS X. Indeed, one concrete scenario would be someone with a used PC that's just one or two years old running either Linux or OS X who decides to convert it to Vista and buys the upgrade version of the OS to do so.

Such users may argue that they are such good customers of Microsoft that they should be allowed to save a little bit of money when Microsoft goofs, like when a retailer is forced to honor a misadvertised price.

But Microsoft remains officially adamant. The spokeswoman noted that customers buying and installing an upgrade version of Vista onto one PC also forfeit the right to use XP on another PC, unless they own more than one full retail copy of XP.

"This is part of the end user license agreement the customer consents to by purchasing a retail upgrade version. We believe it strikes a fair balance for our customers, since upgrade versions allow them to purchase Windows Vista at significantly reduced prices," wrote the spokeswoman. Only customers who pay for full retail versions of Vista "maintain the right to install their previous versions of Windows."