If you have installed Tomcat without the "Start Menu Items" on Windows it may make it harder to increase max heap size.
An approach to help correct this would be:
Back up all current custom configuration and web applications, etc.
Uninstall the old service using the command line: http://tomcat.apache.org/tomcat-5.5-doc/windows-service-howto.html
Reinstall Tomcat, with "Start Menu Items" selected. You might encounter an error when the installer tries to install the service again. You can elect to ignore it.
After Tomcat is reinstalled, you can use the Java tab in "Start" --> "Apache Tomcat 5.5" --> "Configure Tomcat" to increase the heap size.