I just installed Windows 7 on my non-PC and was struck by how many times the Windows installer reboots during installation. It even tells you on the screen during installation that the computer will reboot multiple times.

I know Microsoft must have a good reason for this, but I'm completely baffled why this is necessary. Installation simply writes to the disk, and because the system has booted off the DVD, it already has full write access to the target hard drive partition.

Is Windows in fact installing a preliminary disk image and then booting off of that instead of the DVD in order to speed up installation? (Though that then raises the question, why two reboots?)

It probably seemed like more than that because I had to do an extra reboot initially (back to my host OS) to fix the format of my partition, and then there were the subsequent reboots for Windows updates.


Actually, there are only two reboots during the entire Windows 7 installation process:

Windows 7 prepares the target installation media (hard drive or SSD), formats if necessary, and copies the base installation files for a quicker install, and decompresses them. This also installs the basic Windows Kernel for use in the next step of the installation.

First Reboot

After rebooting, the system is boots into the Windows 7 kernel. Here, all of the packages and drivers are installed. Once this step is completed, you need to reboot the system to Windows will reload all of the newly installed hardware and drivers that it detected during the install. This also lets essential Windows components be activated and installed as services that start upon boot.

Second Reboot

You now have a fully installed version of Windows 7 on your hard drive, with all necessary drivers (motherboard, disk interfaces, controllers, etc...). All final installation processes (OOBE, performance indexing, and cleanup) is done. The system does not reboot after this point, since the install is now technically complete.

As for why this is necessary, DVD media is very slow compared to a hard drive. This is why the installation files are actually copied to disk before they are even decompressed. Furthermore, to maintain compatibility with a large number of systems, a reduced system kernel is used to start the installation, which afterward, the drivers specific to your system are used to boot up after the second reboot.

This is similar to the Linux installation process, with the exception of a few more reboots (most Linux distributions can perform the system preparations and driver installations in one step, eliminating the need for the second reboot). Note that the number of reboots during the installation has remained the same since Windows 95 (the only exceptions being if an error occurs, where you may experience more reboots then necessary).

  • What does Linux do differently which saves the reboots? Is it just that Windows hasn't invested as much into optimizing installation? – Xodarap Mar 11 '11 at 22:51
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    It has less to do with optimization and more to do with how the kernel is structured. Microsoft does it for a variety of reasons, both for speed of installation, stability, and security. It's just how the operating system evolved, and is not a way of making another operating system "better" then another. – Breakthrough Mar 11 '11 at 23:26
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    @Xodarap Generally speaking the vast majority of linux drivers are modular and can be inserted into the kernel while its running which allows it to avoid needing to reboot. Only when the parts of the kernel that are not modular are modified does it need to reboot. This is basically the kernel core and certain baseline drivers. There is even Ksplice now with allows most kernel core updates to happen while its running. – Mark Mar 12 '11 at 17:25

My understanding of the Windows installation process with W7:

  1. Boot From CD, Partition, Copy base system
  2. Reboot
  3. Boot from HD, Expand base system and set up registry and drivers for devices it supports out of the box
  4. Reboot
  5. OOBE (Out of the Box Experience) Where it asks for username, and license keys

Just installed it into a clean VM, only counted two reboots, I'm not sure where the 3+ figure is coming from.

  • Thanks for answer! Upvoted as well, wish I could mark multiple answers as correct. – skue Mar 11 '11 at 18:21
  • No worries, I try to go with brevity when possible, but sometimes the full answer is better. – Nathaniel Bannister Mar 11 '11 at 19:06
  • @skue: One can accept more than one answer. – harrymc Mar 12 '11 at 20:33
  • @harrymc: If so there' a way, it sure ain't intuitive... clicking the checkbox in front of one answer toggles it off for the other. – skue Mar 13 '11 at 1:47
  • Sorry, it used to be this way. I'm then not up-to-date on the rules. – harrymc Mar 13 '11 at 9:27

The first reboot is indeed because it's running on the install media and is switching to the new minimal installation on the hard drive. Subsequent reboots are because many system files can only be updated during OS boot; this is intended to help prevent viruses from overwriting them.


More than 2 reboots often mean that you have an illegal copy of the OS that in infected by malware.

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