Do you have single player worlds? if not, maybe try deleting your minecraft.jar and redownloading it. NOTE: THIS WILL DELETE EVERYTHING IN YOUR MINECRAFT, USE WITH CAUTION.
You also could do this:
Open “Run”, type in %appdata%
place Minecraft.jar into your .minecraft folderCopy the path to your minecraft.jar (It should be along these lines: C:\Documents and Settings\User\Application Data\.minecraft\Minecraft.jar)
Open notepad
Copy and paste this in it: java -Xms650m -jar “…Application Data\.minecraft\Minecraft.jar”
Then Save As: minecraft.bat (Make sure to change the filetype from .txt to .bat and make it save under all Files)
Then you should be able to run minecraft. If you have any questions at all, feel free to contact me. Thanks:D
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