considering its about impossible to buy a pc without paying for a windows license bundled, I would be surprised if the percentage of illegal copies was larger that 0.X%
1) Computer repair shops very frequently format computers and re-install pirated copies of Windows. Considering the spyware problem of not too long ago, this is incredibly common.
2) Many parts of the world don't have stores like Best Buy and Dell might not ship there. Many computers are sold as brand new with a pirated copy of Windows.
3) In some nations, pirated Windows copies are sold by street vendors right next to pirated movies and video games. If you don't understand or respect intellectual property rights, you might as well upgrade from XP to Win 7 for next to nothing.
