Command to change tomcat server settings in tomcat

Open the path till bin and execute the command //ES//alfrescoTomcatnum2
C:\Alfresco3.4.3\tomcat\bin>tomcat6w.exe //ES//alfrescoTomcatnum2

Here alfrescoTomcatnum2 is the name of the service. You can change this name based in your service name.
Now set the required memory settings in the Java tab

Initail Memory pool 256 MB
Maximum memory pool 512mb
Thread stack size 64

If memory is not set then alfresco service may not start