Minecraft doesn't work if there is a "!" in the System username

Solution 1:

Assuming you are running Windows Vista/7. The procedure would work for other versions of Windows but the PATHs will be different.

• Create a folder named C:\Users\Public\Minecraft
• Download the Minecraft.exe file and save it into that folder
• Create a batch file named start-minecraft.bat (or whatever you want) and save it on your desktop or wherever you want to click to start Minecraft
• The contents of the batch file should be

start-minecraft.bat

@echo off
:: set the app data to be a path that doesn't break minecraft
set APPDATA=%PUBLIC%\Minecraft
:: start up minecraft
%PUBLIC%\Minecraft\Minecraft.exe

Basically we just need to set the APPDATA variable to a directory that Minecraft will consider to be valid.

P.S. This same procedure can be used to keep multilpe different versions of Minecraft installed. Just create a different folder for each version, and update the APPDATA variable to reflect that.