According to Microsoft's price list, U.K. customers will pay up to 104% more for an Anytime Upgrade, depending on the upgrade chosen. A Starter to Home Premium Anytime Upgrade, for example, is priced 34% higher in the U.K., when British pounds are converted to dollars at the current exchange rate. (U.S. prices include sales tax -- the average combined sales tax burden in the U.S. was 8.6% in 2008 -- for a better comparison with Europe's Value Added Tax, or VAT. That consumption tax is included in Microsoft's U.K. and EU prices.)

The biggest gap between U.S. and U.K. prices is for the Home Premium-to-Professional Anytime Upgrade; U.K. buyers will pay more than twice as much. For a Professional-to-Ultimate upgrade, however, the two market's prices are essentially a wash.

Prices listed in Euros show an even greater disparity when converted to dollars. The Home Premium-to-Professional Anytime Upgrade costs 163% more in France and Germany, for instance, than in the U.S., while the Starter to Professional is 111% more. The most likely scenario, using Anytime Upgrade to bump up from Starter to Home Premium, costs 23% more in the EU.

Computerworld also compared U.S., U.K. and EU prices for Anytime Upgrades by calculating their cost as the percentage of the price of a popular netbook, the ASUS Eee PC 1000HE, which on Monday was the best-selling netbook on the U.K. version of Amazon.com. The Eee PC is equipped with Windows XP Home, not Windows 7 Starter, as that's not yet available, but analysts expect that the lowest-priced netbooks will run Starter. For its part, Microsoft has for the machines it will allow computer makers to ship with Starter; the ASUS machine meets those Microsoft specifications.

As a percentage of the Eee PC 1000HE's purchase price -- $382.47 on Amazon -- Anytime Upgrade prices in the U.S. ranged from 23% (for Starter-to-Home Premium) to 47% (for Starter-to-Ultimate).