Try this:
Open IE | Tools | Internet Options | Programs tab | make sure
"Internet Explorer should check to see whether it is the default browser" is
CHECKED.
Or:
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.
1.. On the Tools menu, click Internet Options.
2.. Click the Programs tab.
3.. Click the Reset Web Settings button.