Problem:
I start the client, press enter batmud and then nothing happens at all and the login screen(username and password -thingy) remains on the screen, the client doesn't respond to anything at all and the only way to shut it down is from taskmanager.

This has happened once before, and at that time I tried the uninstall client clear cache blabla reboot, reinstall, still same problem.
After that I uninstalled whole java, reboot, reinstall, reinstall client and then the client started working again.

Now, if someone has some ideas about a better solution or why this hangup happens(apparently something to do with java but what?), I'd be happy.

Java 6 update 11, windows xp sp3.

I've had similar problems and I'm not going to claim to have the ultimate answer, but it seems to occur when the automatic Java update thingy happens (perhaps when using the client). I have had success by going into the Java control panel and performing a manual update. Once updated, the client has returned to normal.

I had issues with this happening when trying to load custom scripts or add-ons.

Restoring the /scripts directory cleared this up for me.