IE 5.01 contains a new option (Tools / Internet Options / Advanced tab) labeled Reuse windows for launching shortcuts.
Prior versions of IE had a Browse In A New Process, which is replaced with this new option.
Reuse windows for launching shortcuts is enabled by default.
When enabled, an existing instance of IE is used when you:
Double-click a .url file from within IE.
Open a url in Start / Run.
Use a desktop toolbar, such as the Address or Links bar.
NOTE: Favorites and Links always use an existing instance.
The registry entry that controls this option is:
HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\AllowWindowReuse
The entry is not present unless you toggle the option and press Apply.
You can Add Value name AllowWindowReuse, as a type REG_DWORD, and in the
DWORD Editor, set the data value to 0, to disable this behavior, or 1, the default,
to enable it.
