Microsoft chief executive Steve Ballmer said Tuesday that the company has seen early success with its new operating system, Windows 8.

Speaking at the company’s BUILD developers conference, Ballmer said that there have been over 4 million upgrades to Windows 8 since the company unveiled the operating system on Friday.

According to Ars Technica, that is faster than the upgrade rate for the company’s previous operating system, Windows 7 — though the site noted that upgrading to the previous system was more costly.

Windows 8 upgrades cost $40 online, though those who’ve bought a qualifying PC can upgrade to the system for just $15.

Ballmer also said that Microsoft is working quickly to build out its Windows store, the report said, and will soon include applications from Twitter, Dropbox and ESPN.

Microsoft has promised the store will have 100,000 apps in its Windows Store by January. While an impressive number, it still pales in comparison to the 700,000 apps offered on the platforms for Apple and Google devices.

Windows 8 is built to work on touch-enabled and traditional desktops and notebooks. For the new system, Microsoft replaced its Start button with a Start screen that showcases a users’ most frequent or favorite apps, as well as live notifications from their e-mail accounts and social networks.

Related stories:

Windows 8: What to know before you upgrade

VentureBeat: Windows 8 stuns, amazes, and sometimes lets us down (review)

Microsoft Surface in stores today: what to know before you buy