> register themselves in HKEY_LOCAL_MACHINE\SOFTWARE\
> Clients\StartMenuInternet so that users can change their 
> default browser through the OS. On 64-bit systems, this 
> is always in the 64-bit registry

According to "Registry Keys Affected by WOW64", the "Clients" key is redirected in Vista and shared in Windows 7.

https://msdn.microsoft.com/en-us/library/aa384253

    import winreg

    HKLM = winreg.HKEY_LOCAL_MACHINE
    subkey = r'Software\Clients\StartMenuInternet'
    read32 = winreg.KEY_READ | winreg.KEY_WOW64_32KEY
    read64 = winreg.KEY_READ | winreg.KEY_WOW64_64KEY

    key32 = winreg.OpenKey(HKLM, subkey, access=read32)
    key64 = winreg.OpenKey(HKLM, subkey, access=read64)
    # This should be true in Windows 7.
    assert winreg.QueryInfoKey(key32) == winreg.QueryInfoKey(key64)

I don't have a Vista installation to confirm that it's really redirected. If so, it's important to enumerate the WOW64 redirected key as well, since 32-bit browsers are still common on Windows.