Try setting your HQ_JAVA_HOME to the path of a 64-bit Java JRE on the system. You'll need to then uninstall the Agent service and reinstall it to make it pick up the new environmental variable (which is somehow stored in the Windows Service configuration.)
Not sure if this will work or not, since SIGAR is a 32-bit application/library, that is not written in Java, and is part of the agent. But it is worth a try. Please let us know if it works.
Brian