How to Reinstall Windows

By Jack Gerard

Installed on approximately 84 percent of personal and business computers, Microsoft Windows is the most widely used computer operating system in the world. Virus infection, file corruption and other problems can sometimes result in damage to the operating system that requires reinstallation of the OS to correct. Even if the operating system is still functional, you may decide to reinstall as a way to correct registry errors, software compatibility issues and general OS bloat that is caused by installed but unused Windows components.

Things You'll Need

  • Windows installation disc


Step 1

Perform a virus and malware scan on your computer before reinstalling Windows. While the reinstallation process overwrites any infected system files, other files on your computer could be infected. Run a scan before you reinstall to eliminate the infections or remove infected files so that they can't reinfect your system after you've completed the reinstallation. When the scan is complete, turn off your anti-virus software before beginning the reinstall and leave it off until after the process is finished.

Step 2

Make backup copies of your documents, pictures and other important data by copying the data to a flash drive, external hard drive or an internal hard drive partition that is separate from your Windows partition. If these options aren't available, use removable media such as writable DVDs to back up your files. Locate your Windows product key and the product keys for other software you will reinstall; the Windows key should be included with the installation disc or attached to your computer if it came with Windows installed on it. Write down details such as your user name, computer name and workgroup name to make sure that they are entered correctly after you complete the reinstallation.

Step 3

Determine how you wish to reinstall Windows. Two options are available to you: a full reinstallation using the "Custom" installation option or an "in-place upgrade" reinstallation using the "Upgrade" installation option. A full reinstallation gives you the option to format the drive partition before you reinstall, although formatting is not required. An in-place reinstallation overwrites system files in the same way that upgrading to another version of Windows would and does not reformat the drive. If you decide to format the drive, you will choose between a full format that securely removes all information from the partition and a quick format that overwrites the previous data. The full format ensures that all previous data is completely removed, but it takes much longer than a less-secure quick format.

Step 4

Uninstall any Service Pack updates that you have previously installed if you do not plan to format your Windows partition. This is done by clicking the "View Installed Updates" link in the "Uninstall a Program" window, selecting the Service Pack update you wish to remove and clicking the "Uninstall" button. Windows may not reinstall correctly if the Service Pack updates aren't removed before beginning the installation process since the software will see the reinstallation process as an unauthorized downgrade.


Step 1

Insert the Windows installation disc into your computer's DVD drive. If you wish to perform an "in-place upgrade" reinstallation, run the "setup.exe" file from the disc while Windows is running. If you plan to perform a full reinstallation, reboot the computer and press any key when prompted to boot from the disc in the DVD drive.

Step 2

Click the "Install Now" button on the initial installation screen. Choose the option to download updated installation files to avoid installation problems and then read and accept the Windows user agreement. If you are performing an in-place upgrade reinstallation, click the "Upgrade" option when prompted to select the type of installation you want to perform. If not, click "Custom (advanced)" to begin a full reinstallation.

Step 3

Select the drive where you wish to install Windows from the list of hard drives generated by the installation program. A list of installed Windows versions appears instead if you are using the "Upgrade" installation option. After a drive is selected, you have the option to format that drive using one of the format types discussed previously by clicking the "Drive Options (advanced)" link and selecting the format option you want to use; this link can be ignored if you don't want to reformat the drive. Click "Next" when you are ready to begin the reinstallation.

Step 4

Input your Windows product key when prompted; this allows the installer to select the appropriate files to install for your version of Windows, since the Windows installer contains the files necessary to install every Windows version. Confirm that the "Activate Windows When I'm Online" box is checked to ensure that Windows is activated properly unless you plan to activate the software manually when reinstallation is complete.

Step 5

Wait while the Windows installer reinstalls all the system files for the operating system, a process that can take up to half an hour or longer depending on your computer's hardware. Enter your name, a computer name, your time zone and the security setting for your network when prompted to complete the installation.

Step 6

Reinstall any programs that were deleted during the format and reinstallation; if you performed an in-place upgrade reinstallation then this will not be necessary. Copy your pictures, documents and other backed-up data back to your hard drive when you've finished reinstalling your other software.

Tips & Warnings

  • When you perform a full reinstall of Windows without formatting the Windows partition, all previous user files and system files are saved in a folder named "Windows.old." Software will still need to be reinstalled to ensure proper functioning, however.
  • Don't forget to download new drivers for hardware such as video cards, sound cards and custom keyboards. Your hardware may not function correctly until the proper drivers are installed, and downloading the latest drivers from the manufacturer's website ensures that your driver software has the latest fixes and features.
  • File corruption and a damaged boot sector can make it impossible to perform an in-place upgrade reinstallation. In this case, choose to reformat your Windows partition before proceeding with a full installation.