According to a new survey by NPD Research, cited on Gizmodo.com and Macrumors.com, Apple’s penetration of the U.S. personal computer market is up from 9 to 12 percent. So do you think Steve Ballmer and company are sweating it in Redmond? Not really. PCs running Microsoft Windows still account for 85 percent of the national market share. And what’s worse for Apple? The dreaded “mixed system environment,” meaning households that have both Macs and PCs.If you look at Mac-only households, that’s only 2 percent of the market! In other words, Apple’s gain against MSFT is coming on the ancillary side–the second computer that sits in the phone nook by the kitchen, or for your kid before he or she goes to college. That shiny new Mac is like a designer lapdog you get to keep the family’s faithful golden retriever company. So it’s hard to argue that Apple’s growth is coming at Microsoft’s expense, when both brands can coexist peacefully in the same household. Growth is merely overlap. And Ballmer can giggle at Apple’s puny growth in market share, which still looks like this: