Microsoft's offer to sell home users extra licenses to use their Windows XP software on additional PCs is a hit. It's such a big hit, you can't find the licenses anywhere. Windows XP's product activation, which essentially ties the copy of the operating system to the PC on which it's installed, is intended to stop the illegal practice of installing the OS on multiple PCs. To further encourage multiple-PC homes to play by the rules, Microsoft offers additional licenses for slightly less than the cost of the initial retail package (the offer doesn't apply to owners of new PCs with Windows XP). The problem: A month after Windows XP's release, Microsoft has already run out of licenses. "There is a backlog," says Mark Croft, lead product manager for Windows XP. "What happened is we made some projections about how many additional family licenses we would sell in the first 90 days, and the demand has far exceeded our expectations." Microsoft won't give figures, or even reveal the percentage of sales that are family licenses. "We're in the process of restocking, but while it sounds like it may be easy, there is actually a manufacturing process involved," he says. That process involves the creation of a paper license and a new product key code. Croft says he expects the supply of licenses to improve by the end of November.