As mentioned earlier this year, Windows 10 will be a free upgrade for anyone with Windows 7 or Windows 8.x so it's reasonable to assume it will capture a lot of marketshare quite rapidly as many people, and especially gamers, will want to upgrade.
We got a better date than that as our sources are promising that we will have Windows 10 in July. All customers with legal versions of Windows 7, 8 and 8.1 can upgrade. We have been playing with Windows 10 since the first Technical Preview and we we like it. It is a synergy of best from Windows 8 and important elements like desktop inspired by Windows 7. Many Window 7 users will want to upgrade.