O Internet Explorer usa somente seu executável de 64 bits quando a configuração do navegador "Modo Protegido Avançado" está habilitada na guia Segurança. Caso contrário, o executável de 32 bits é usado e o JRE correspondente de 32 bits deve ser instalado:
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In Internet Explorer on the Desktop, by default, Content Processes remain at 32bit by default for compatibility with 32bit ActiveX controls, Toolbars, BHOs, etc. Even when you directly launch the 64bit iexplore.exe executable, you will still have a 64bit Manager Process that hosts only 32bit Content Processes. If you want to enable 64bit Content Processes for the Desktop, you must tick the Enable Enhanced Protected Mode option in the Security section of Internet Explorer’s Tools > Internet Options > Advanced tab. When this option is enabled, all Content Processes that are running in Protected Mode (e.g. Internet Zone and Restricted Zone, by default) will begin to use 64bit Content Processes.