As far as I know, there's no registry key or setting anywhere in Windows® that says specifically what the browser is. But then, the full mysteries of the registry are deeper than any individual can probe, even supposed experts. For all I know, there could be a key somewhere, HKCUSystemMumbleBletchBlahGakDefaultBrowser. If you know of such a key, please write in. In the meantime, the quickest way I know to solve your problem is to just look at which program is associated with HTML files. In Windows, HTML files have an extension of .htm or .html, so all you have to do is look in the registry under HKCR/.htm. If you do, you'll find something like:
HKEY_CLASSES_ROOT.htm="htmlfile"
And if you follow this to HKCR/htmlfile, you'll find something like:
[HKEY_CLASSES_ROOThtmlfileshellopencommand]
@=""C:\PROGRA~1\INTERN~1\iexplore.exe" -nohome"
In this case, Microsoft® Internet Explorer (iexplore.exe) is the program used to open .htm files. (The -nohome switch tells Internet Explorer not to open your home page.) If the default browser were Netscape, the entry would be something like:
[HKEY_CLASSES_ROOThtmlfileshellopencommand]
@=""C:\PROGRA~1\NETSCAPE\netscape.exe".
Got it?