The default browser, as such, is not disabled. You must specify the correct settings in Internet Explorer that you want to set as a default browser. Also, if you remove one of the programs, the second will automatically become the default browser.
To assign the default browser, Internet Explorer, run it and in menu "Tools", open "Internet options". Make it the active tab "Programs". Look in the "Program overview" default button "Use default" and click on it with the left mouse button. If you want each time you launch the browser validation has been performed, check the marker box "Tell me if Internet Explorer is not the default". Save the new settings by clicking on the "Apply" button and close the window.
If the computer has extras installed Opera browsers, when you first start it, a window will appear with a proposal to make a program your default browser. Answer Yes, or launch application and in the Opera menu, select "Settings" and the sub-item "General settings".
This will open a new dialog box. Go to the "Advanced" tab. Set the marker in the field "to Verify that Opera is the default browser" and click on the "Settings" button to the right of the row. In the additional window, select the marker item "default Browser", confirm new settings by pressing OK button.
In order for the default browser was the Mozilla Firefox application, run it and in menu "Tools" click on "Settings". In the new dialogue window, go to the advanced tab and make active mini "General" tab. Click on the "Check now" next to "Always check at startup if Firefox is the default browser". After verification you will be asked to make the app your default browser. Click in the query window, click "Yes."