If you're using XP, give this a try.
DefaultBrowser allows you set the default web browser for your user profile in Windows XP, without affecting the other users. The list of browsers you see in the Presets are tested and working flawlessly as user defaults. Note that the user interface in Windows XP does not provide an option for setting the default browser for each user.
This utility is designed for Windows® XP systems only.
Apart from the Presets list, there is an option at the bottom which allows you choose the Web Browser program file and set it as the default. You may use this section if your favorite browser is not listed under Presets. As you read in the dialog, this method does not work with all the browsers. If the results are not satisfactory using this method, you can undo the configuration by clicking the Reset button.
Here is the list of Web browsers that I've tested (and working) for the per-user feasibility, and their versions: (continued)....http://windowsxp.mvps.org/defaultbrowser.htm