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> 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.