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If you use IE, go to Tools -> Internet Options -> Programs, then click "Reset Web Settings." Uncheck the "Also reset my homepage option" and click okay.
Alternatively, if you've customized your IE settings for some reason and don't want to lose those, just check the "Internet Explorer should check to see whether it is the default browser" option box and restart your browser. Click "Yes" when it asks if you want to set IE as your default browser.