The most likely cause
of a "Browser not found" issue is that the "Web Loader Class" add-on is
disabled in IE.
(Go to Tools → Manage Add-ons to enable.)
Another possibility is
that IE8 is being used and Compatibility View is disabled. Make sure that Compatibility View is being used for all pages.
(Go
to Tools→Compatibility View Settings and check the "Display all websites in
Compatibility View" checkbox.)
The third possibility is that the web site domain
is not specified in the Trusted Sites zone.
(Go to Tools → Internet Options → Security tab, select "Trusted Sites",
then click the Sites button and Add the site.)
Please also see the artcile: Browser not found after Load Browser step on 64 bit Windows OS.