Biggest competitor of Microsoft for its Windows 7 is ... Microsoft!
"One fairly universal complaint is that Vista is bloated and needs a lot more hardware to make it work well compared to XP. Microsoft tacitly acknowledges this problem ..."
"It's bloated and top-heavy, with an insatiable appetite for state-of-the-art hardware. Basically, it chews up CPU and memory capacity like it's going out of style. .. What is it, exactly, that Windows 7 does so much better than its leaner, meaner, pre-Vista ancestors?"
"Windows 7's biggest competitor will be Windows XP" (Joe Wilcox) ... "Microsoft's last operating system, Vista, was so poorly reviewed that many people simply stuck with machines running the eight-year-old Windows XP system" (Phil Schiller). .. Perhaps Windows 7's competition isn't so much XP as it is apathy."