I'd look at your PATH and JAVA_HOME environment variables. If they refer to a JRE directory, change that to the JDK directory. Then start a new shell and see if "java -version" has changed.
In the meantime, if you manually invoke the full path to your JDK's java.exe with the -version option, you can see if that one's even a server VM to begin with. :-p If it isn't, maybe try "C:\...\java.exe -server -version" to be sure.