As far as I'm concerned, Linux will never likely replace an OS such as Windows XP or Windows 7 but you can't blame Canonical for trying. In their latest public push to get people to move away from Microsoft, they are showcasing 14.04 LTS (Long Term Support) as the perfect product at the perfect time to replace your aging desktop OS. I do wish them luck, but until you can use Windows-compatibel programs on Linux, you're only going to get a very small install base.
The 14.04 LTS release offers a solid, intuitive experience which is easy to manage," Canonical CEO Jane Silber said in a statement. "It is a viable and affordable alternative for those organizations considering a switch from Microsoft, and specifically those replacing XP or Windows 7 as they come to the end of life.