"Small Business. If you operate a small business, then you may install and use the software on up to ten (10) devices in your business."
FTS: "About ten boxes here are still running Windows XP and would be too old to upgrade to any newer version of Windows."
Suggesting that they have other boxes that are not running XP and not too old to upgrade.
Counting his own machine, that brings the sure total to at least "About" 11.
Though the EULA doesn't actually define how big a "small business" can be.. So if he's just at about 11, or slightly more, then yeah, he can use it, but if his company is too big to be considered a "small business" then it is a no go for even those 10. How big is too big? Who knows. (Bring on the phallus jokes...)
Sounds like the OP has at least 10 PCs, so I figured I'd throw this out here.
Need I remind you that USA revolted from a European country in 1775?