Symptoms

Confluence refuses to start with the following error message:

Error occurred during initialization of VM
Could not reserve enough space for object heap
Could not create the Java virtual machine.

Cause

The Xmx (heap space memory) is set beyond what a 32-bit JVM can handle.

Resolution

Set the heap space memory lower. See Fix java.lang.OutOfMemoryError in Confluence.
If you need to use a bigger heap space, you should migrate to a 64bit OS with 64 bit JVM.
For Windows 64bit, you should substitute the tomcat6.exe and tomcat6w.exe binaries. Please refer to Problems installing Confluence as a service on Windows 64bit.