Well, could not find class: Main, means that minecraft.jar isnt in the lib folder.

I know you said you did this, but double check for me. ;)
ALSO! Alpha .jar's do not work.  so make sure to download the non-alpha.

Next, Make sure you have downloaded the latest JDK's (Java).

Google will help you find the download links. ;)

After you have done all that, run, run.bat as administrator.

This is done by rightclicking run.bat and clicking "run as administrator"

-Best of Luck!

I have recently had a error similar to this, I have found a solution that works for me and hopefully you to. Make a folder on your desktop ( Make sure you have it on your desktop), then put all the files that were in the wom client in the folder. Now put your minecraft.jar file in the lib folder now run it, it should work. Good Luck.

I've asked every player I know about this, told them that my Client is installed EXACTLY as it is mentioned with the .Jar file in the lib folder, and the ONLY advice people can give is "You didn't put the .jar file in the /lib folder" At this point its gun-to-the-head frustrating..

Having exactly the same issue. Running Windows 7 64-bit with the latest Java updates and the 1.5.9 client download, and the .30 of the Minecraft.jar.

SO. Triple checking all the files are in their correct places, I start-up run.bat and low and behold THIS error message pops up:

Starting java from "C:\Program Files (x86)\Java\jre6\bin\java.exe"
Exception in thread "main" java.lang.NoClassDefFoundError: com/mojang/minecraft/
MinecraftApplet
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClassCond(Unknown Source)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$000(Unknown Source)
        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)
Caused by: java.lang.ClassNotFoundException: com.mojang.minecraft.MinecraftApple
t
        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)
        ... 12 more
Could not find the main class: Main.  Program will exit.
ERROR: Failed to run minecraft
JAVA6="C:\Program Files (x86)\Java\jre6\bin\java.exe"
Press any key to continue . . .

Okay. I've done everything right... now I go delete all my temp folders for Java, and both my browsers. Guess what? no change....

Does anybody have a decent answer for this?

I have the same issue I think.  Have any first timers got this to work in the past week?

edit:  darkterras posted the link to the old jar in another thread: http://www.worldofminecraft.com/node/9716     This worked for me!

I've got the Alpha jar, didn't work.

It is really hard to find an answer to this "bug". Don't know what's going on, but it's annoying.

I get the same error, I'll copy paste it below, and I also tried the link in this topic: http://www.worldofminecraft.com/node/9716#comment-16639 that was mentioned earlier, but it didn't work.

If anyone has any idea what this might be (maybe a Windows 7 issue?) please, do post!

-----------------

Autodetecting java
Testing default folder
Checking C:\Program Files\Java\jre6 folder
Found java in C:\Program Files\Java\jre6 folder
Starting java from "C:\Program Files\Java\jre6\bin\java.exe"
Exception in thread "main" java.lang.NoClassDefFoundError: com/mojang/minecraft/
MinecraftApplet
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClassCond(Unknown Source)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$000(Unknown Source)
        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)
Caused by: java.lang.ClassNotFoundException: com.mojang.minecraft.MinecraftApple
t
        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)
        ... 12 more
Could not find the main class: Main.  Program will exit.
ERROR: Failed to run minecraft
JAVA6="C:\Program Files\Java\jre6\bin\java.exe"
Press any key to continue . . .

I did what you said Automated Chip but then it just added an error with the did you read the instructions thing :/
thanks for the help because you helped me once but now its not working.