Downgrading your Minecraft client from "1.2.4" to "1.2.3":

1 - Open up the folder "C:\Users\Username\AppData\Roaming\.minecraft\bin"
2 - Find a file called "Minecraft.jar"
3 - Rename the file to "Minecraft-1.2.4.jar"
4 - Find a file called "Minecraft-1.2.3.jar"
5 - Rename the file to "Minecraft.jar"

Done
- Xaysia