See Run RabbitMQ as a Service on http://www.rabbitmq.com/install-windows.html.
Environment variables can be used but that requires service *reinstallation*
(not just restart):
http://www.rabbitmq.com/configure.html#customise-windows-environment
RabbitMQ: Could not start erlang service "RabbitMQ"with commandline ["C:\Program Files\erl6.1\erts-6.1\bin\erl.exe" -service_event ErlSrv_RabbitMQ -nohup -sname rabbit@WIN7X64ENT-0 -pa "C:\\Program Files (x86)\\RabbitMQ Server\\rabbitmq_server-3.3.4\\sbin\\..\\ebin" -boot start_sasl -s rabbit boot -config "C:\\Windows\\system32\\config\\systemprofile\\AppData\\Roaming\\RabbitMQ\\rabbitmq" +W w +A30 +P 1048576 -kernel inet_default_connect_options "[{nodelay,true}]" -sasl errlog_type error -sasl sasl_error_logger false -rabbit error_logger {file,\""C:/Windows/system32/config/systemprofile/AppData/Roaming/RabbitMQ/log/rabbit@WIN7X64ENT-0.log"\"} -rabbit sasl_error_logger {file,\""C:/Windows/system32/config/systemprofile/AppData/Roaming/RabbitMQ/log/rabbit@WIN7X64ENT-0-sasl.log"\"} -rabbit enabled_plugins_file \""C:/Windows/system32/config/systemprofile/AppData/Roaming/RabbitMQ/enabled_plugins"\" -rabbit plugins_dir \""C:/Program Files (x86)/RabbitMQ Server/rabbitmq_server-3.3.4/sbin/../plugins"\" -rabbit plugins_expand_dir \""C:/Windows/system32/config/systemprofile/AppData/Roaming/RabbitMQ/db/rabbit@WIN7X64ENT-0-plugins-expand"\" -rabbit windows_service_config \""C:/Windows/system32/config/systemprofile/AppData/Roaming/RabbitMQ/rabbitmq"\" -os_mon start_cpu_sup false -os_mon start_disksup false -os_mon start_memsup false -mnesia dir \""C:/Windows/system32/config/systemprofile/AppData/Roaming/RabbitMQ/db/rabbit@WIN7X64ENT-0-mnesia"\" -kernel inet_dist_listen_min 25672 -kernel inet_dist_listen_max 25672 ].