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


If opera asks to set it as a Default Browser Click no,For the permanent solutionopy

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.