Although the update has been prepared for Windows XP, Vista and 7, not all users will see it.
It will not pop up in front of those who already run a different default browser, such as Firefox, Safari or Chrome.
Users guaranteed to see it are those that have Internet Explorer (IE) set as their default browser and have taken the option to let Windows automatically download and install updates.
Those that have this option turned off will be prompted to download, install and run the software.
Those that have automatic updates turned off can go to the Windows Update site and run a "Check For Updates" to get it.
When it runs, the software will present users with a window that says in bold text "An important choice to make: your browser". It also asks people to ensure they are connected to the internet.
Once the "OK" button is clicked, they will see a screen that lists the 12 browsers available.
The Opera, Firefox, Chrome, Safari and Internet Explorer browsers are randomly ordered on the first section of this screen.
The browser choice comes as part of an anti-trust deal
Another seven browsers, namely Sleipnir, Green Browser, Maxthon, Avant, Flock, K-meleon, and Slim, will be randomly ordered on the rest of the screen. They can be viewed by scrolling sideways.
Below each listing will be a button giving more information. Another button lets users install one of the browsers.
Alternatively, users can opt to ignore the selection and make a choice later. If they do this a shortcut icon will appear on the Windows desktop that gives access to the browser choice window at any time.
The choice screen will keep appearing until a user has made a choice.
Anyone wanting to stick with IE will be prompted to upgrade to the latest version.
Microsoft said that Windows 7 users who have fixed or "pinned" IE to their taskbar will have it unpinned by the update. Right clicking the icon for any browser will let users pin that program to the taskbar.
It is not yet clear what effect the browser choice will have on Internet Explorer's market share.
Although exact estimates are hard to find, web stats firm Net Applications said IE is used by 62% market share. Its closest rival is Firefox at 24%.
Some fear the browser choice system will confuse people. Already some who have been confronted with the choice screen have been worried that their machine has been taken over or fallen victim to a virus.
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