Got a large business? Buying lots of PCs this summer? Don't get too excited about the upgrade coupons that Vista-equipped computers are now coming with. Microsoft is limiting the number of free upgrades to which any business is entitled to 25 copies, according to a story in InformationWeek.

The move is decidedly strange and is one which I'm still struggling to understand the rationale behind.

Obviously Microsoft is looking for a little extra cash in the form of getting a few extra paid upgrade licenses, but that's likely to backfire. Rather, the company is likely to simply lose a good chunk of business it might have otherwise earned over the summer as enterprises instead choose to put off hardware purchases until Windows 7 is released in October.

There is, however, one solid theory for the move: Microsoft wants businesses to buy its high-end Enterprise Agreements, which entitle the purchasing company to unlimited upgrades over the next three years... in exchange for fees that begin at $100 per PC.

The move won't affect individual buyers. Consumers who purchase a new PC (or 25 of them, I suppose) equipped with Vista today will receive a free upgrade to Windows 7 when it is released.

InformationWeek suggests that businesses might be able to work around the restrictions by pressing their PC manufacturer of choice for an exception to the rule or other special terms for obtaining upgrade copies. The story suggests that such exceptions are actually rather commonplace... and hints that if your current manufacturer won't offer free upgrades, you might consider taking your business elsewhere.

UPDATE: Microsoft writes to note that the old policy was only five free upgrades per buyer, so this is actually a considerable improvement on that.