I've had this before, when it happens I usually turn down the process priority in task manager for the java.exe process which is using the most CPU (the installer starts two java.exe processes). It might not work for you, but it's worth a shot.
I've not heard of "us-en" being a requirement to install, in fact the default locale I use is the UK one, and I usually uninstall the US locale when I first install the OS. I remember this applied to certain products for LC7, but it's not been something I've had to worry about with ES.
A side issue (probably unrelated), when you say "latest JDK", I think ideally it would be a Java 5 JDK, not the latest major version, 6. From the "System prerequisites" section of installation docs:
Sun Java Software Development Kit (JDK) 1.5.0_11 or a later update of JDK 1.5.0.
Disabling the antivirus before launching the installer speeds up the process. If you have one running might be worth trying that.
If you want to check that the installer is actually unpacking something try FileMon from the Microsoft website. You should see java.exe process writing loads of files into the temporary directory during the unpacking phase... if that's not happening then it is suffering from starvation.
It's worth noting that if you are running the installer across a network, this phase can take a considerable amount of time, and also that having Filemon running will increase this further, so it's worth just opening Filemon for a short time every few minutes just to check something is happening.
You'll want to set a filter up in Filemon to just include java.exe.
In Filemon, if you see that the only file being accessed is one called "splash.flg", then try the process priority trick at that point and observe whether files start being written to the temporary folder. I find it can take 5-20 minutes to unpack across a network depending on network/processor speed, even after disabling anti-virus and adjusting process priorities.
OK, at the point where it is just accessing splash.flag, check you can see two java.exe processes in task manager, look for the one that is using the most CPU, and set the priority to BelowNormal on just that java.exe, then see if files begin unpacking to the temp folder again.
Make sure the path to the installation bits is short. I ran into a problem where the installer mysteriously stopped at the same step every time. For whatever reason the install bits were in C:\Documents and Settings\Administrator\My Documents. When I moved to C:\LC\ I had no issues.
If you are installing the JBoss turnkey the path from which the install was launched and where it is being installed to can not be very long (the JBoss folder is being copied and contains some files with very long nested directory paths, we cannot go over the windows 256 character limit). There is also a memory check requiring 4gb of memory for turnkey operations. There have been issues where the warning message has not been visible and the install appears hung, this has only happened on certain VMware images to my knowledge.