If you're on Windows 10, a (rather outdated) 32-bit version of MuseScore can be installed from the Windows Store. Clicking here will open MuseScore's page in the Store app. There you will only have to click Get the app > and MuseScore will be downloaded, installed and subsequently automatically updated.
Otherwise you can get the Windows installer from the download page of the MuseScore website. Click on the link to start the download (pick your choice of 64-bit or 32-bit). Your Internet browser will ask you to confirm that you want to download this file. Click Save File.
When the download finishes, double-click on the file to start the installation. Windows may prompt you with a security window to confirm this before running the software. Click Run to continue.
The installation process will now start
If you click Cancel, here or later, you'll see:
If instead you click Next to continue, the setup wizard displays the terms of the free software license.
Read the terms of the license, make sure the box next to "I accept the terms in the License Agreement" is checked, and click Next to continue. Next the installer will ask you to confirm the location in which to install MuseScore.
If you are installing a newer version of MuseScore but still want to keep the old version on your computer, then you should change the folder (note that MuseScore 3 can coexist with MuseScore 2 and 1 with no changes needed). Otherwise click Next to continue.
Click Install to continue.
Give the setup wizard a few minutes to install the necessary files and configurations. You'll see
Click Finish to exit the installer. You may delete the installer file you downloaded.
To start MuseScore, from the menu, select Start→All Programs→MuseScore 3→MuseScore 3.
You can install MuseScore silently with the following command
msiexec /i MuseScore-X.Y.msi /qb-
ALLUSERS=1 if you want to install it for all users of the system rather than just the current one.
You can uninstall MuseScore from the menu by selecting Start→All Programs→MuseScore 3→Uninstall MuseScore; or via Windows' Control Panel. Note that this will not remove your scores, templates, extensions and plugins nor your MuseScore settings.
The installer might be blocked by the system. If you don't manage to install MuseScore, right-click the downloaded file and click Properties. If there is a message "This file came from another computer and might be blocked to help protect this computer", click on "Unblock", "OK" and double-click on the downloaded file again.