I have never had this problem before.

I code all my applications in the Eclipse IDE, and they all work fine there.
But if I go into the bin and use a batch file, with 'java ClassName' it gives me an error about not finding the main class.
I have never had this problem in the past, so I know the command is correct and all that, and I havn't updated Java recently.

Anybody have any suggestions?

Please copy and paste the full text of the error message. There could be important info there.
Is the class in a package?

Exception in thread "main" java.lang.NoClassDefFoundError: Servient
Caused by: java.lang.ClassNotFoundException: Servient
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClassInternal(Unknown Source)
Could not find the main class: Servient.  Program will exit.
Press any key to continue . . .

No, they are not in any packages.
Like I said, the same exact files and everything worked like 2 weeks ago. They've just been sitting on my thumbdrive. And today I plug it back in, run the batch file, and it stopped working.
I thought it was just a code error at first, because the address the bot connects to changed. But I tested other applications on the same thumbdrive and they all spat out the same error.

*edit: Also applications on my hardrive as well, not just the thumbdrive.

NoClassDefFoundError: Servient

I need a bit more info:
Where is the class file Servient.class?
What command do you issue? What's in the batch file?
What directory are you in when you execute the batch file?
Is there a drive letter used in the batch file? >> Your stick is on different drive.

Can you cd to the folder with the class files, issue a dir command to display the contents of the folder and then issue the java command?
Then copy and paste here all that's shown on the console.

Here's what I get when there is no class file:

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

D:\Norms\Norms Tools>java asdf
Exception in thread "main" java.lang.NoClassDefFoundError: asdf
Caused by: java.lang.ClassNotFoundException: asdf
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class: asdf. Program will exit.

D:\Norms\Norms Tools>

Nah. lol, I knew it was none of that.

Turns out it was Quick Time. Quick Time messed everything up for some reason.
I uninstalled it about a week ago because it kept trying to take over my Flash players online.
And I installed it again a while ago because somebody sent me a .mov

So I guess that's a pretty crazy reason, but whatever. That's what it was in case anybody else runs into this same thing.

What did Quick Time do that messed up everything?
How are java and Quick Time related?