Applies to Windows XP - Windows 7 and possibly higher.
There are two types of default browser setting. They are typically set to the same browser. One type of setting controls what browser opens when you click on a web document saved to the hard drive. The other determines what browser opens when you attempt to go directly to a web site via the http protocol such as entering the address in the Run -> Open dialog.
Default browser that opens when html file is clicked
- HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.html\OpenWithList
Default browser that opens when http protocol used (going to web site)
- HKEY_CURRENT_USER\Software\Classes\http\shell\open\command
These are typically both set by your favorite web browser. They can be hijacked by a rogue program or intermediary. Check them via the Registry Editor regedit.exe to manually example their value.