Windows XP only:

DefaultBrowser is a simple utility that allows you to set a portable browser as your system default and easily reverse the process when necessary.

Whatever your reason for wanting to use a portable version of your favorite browser instead of installing it—many people like portable versions because of the extreme ease with which they can back up their entire browser and settings before reinstalling—DefaultBrowser makes doing so a two-click process.

You can select from one of the 11 preset entries or browse to the actual executable. Click Apply and that portable browser will be the system default until you run the application again and hit the Reset button. The process is completely reversible and makes no permanent changes. DefaultBrowser is free and available for Windows XP only.

DefaultBrowser