Now a days people are not sticking to one Browser like Internet Explorer ,they are using multiple Browsers like Safari,Firefox,Opera for the purpose of email they use Internet Explorer to View other sites using Firefox so they are Configuring how to use various browsers for their needs .SO there is automatically no one is set to default so other browsers are compete for supremacy ask to make them as the default Browser.
1.Configuring opera for this
Open Opera click Tools and select preferences and select advanced and disable check if Opera is the Default browser on startup.
Note :opera used here is opera9.27
2.Configure Internet Explorer for this
Open Internet Explore Tools Advanced Internet Options and select Programs and disable Internet Explorer should check to see Whether it is the default browser.
Note :Here IE used is IE6.0
3.Configure Firefox for this
Open Firefox Tools Option and click Main tab and in the System Defaults disable always check to see Firefox is the default browser on startup.