I am getting this error when I copies Android SDK folder from my friend's desktop to mine. I changed the path of my SDK in preferences and it told me to update my ADT which I tried to do but was unable as it gave me errors that it cannot rename my folder name. I also tried to delete my previous SDK folder but failed. Then also I have reinstalled the plugin in my Eclipse Indigo, no success. My dx.jar do exist in lib folder of platform-tools. Can some one help?
This is caused by the existence of an older version of dx.jar in the VirtualStore (a folder where windows stores user versions of system and program files).
Go to "C:\Users\<your user name>\AppData\Local\VirtualStore\<android-sdk path>" and delete all the content, or just the "\platform-tools\lib\dx.jar" file.
i.e: if your user name is "user32" and your android SDK is placed in "C:\Program Files\Java\android-sdk", after the update you'll probably have an obsolete version of dx.jar in "C:\Users\user32\AppData\Local\VirtualStore\Program Files\Java\android-sdk\platform-tools\lib\"
来源：https://stackoverflow.com/questions/15136752/the-file-dx-jar-was-not-loaded-from-the-sdk-folder