To reset Internet Explorer settings
If you installed another Web browser after installing Internet Explorer and Internet Tools, some of your Internet Explorer settings may have changed. You can reset your Internet Explorer settings to their original defaults, including your home page and search pages, and choice of default browser, without changing your other browser's settings.
On the Tools menu, click Internet Options.
Click the Programs tab.
Click the Reset Web Settings button.
Or you could go to Tools > Internet Options >
Programmes tab and tick the box 'Internet Explorer should check to see if it the default browser' , close and reopen IE and it should ask if you want to set IE as the default.