still no solution to throwable error?

doing multiple projects for college.

saved the projects and the enxt time i try to open them, the following pops up.

Unable to load world: S:\alice programming\ninja wa\ninja2.a2w


Alice version: 2.2 09/10/2010

Throwable that caused the error:
java.util.zip.ZipException: invalid CEN header (bad signature)
at java.util.zip.ZipFile.open(Native Method)
at java.util.zip.ZipFile.<init>(Unknown Source)
at java.util.zip.ZipFile.<init>(Unknown Source)
at edu.cmu.cs.stage3.io.ZipFileTreeLoader.open(ZipFil eTreeLoader.java:66)
at edu.cmu.cs.stage3.alice.authoringtool.AuthoringToo l.loadWorld(AuthoringTool.java:1945)
at edu.cmu.cs.stage3.alice.authoringtool.AuthoringToo l.loadWorld(AuthoringTool.java:1912)
at edu.cmu.cs.stage3.alice.authoringtool.AuthoringToo l.showStartUpDialog(AuthoringTool.java:1329)
at edu.cmu.cs.stage3.alice.authoringtool.AuthoringToo l.openWorld(AuthoringTool.java:1361)
at edu.cmu.cs.stage3.alice.authoringtool.Actions$2.ac tionPerformed(Actions.java:92)
at javax.swing.AbstractButton.fireActionPerformed(Unk nown Source)
at javax.swing.AbstractButton$Handler.actionPerformed (Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed (Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.AbstractButton.doClick(Unknown Source)
at javax.swing.plaf.basic.BasicMenuItemUI.doClick(Unk nown Source)
at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mou seReleased(Unknown Source)
at java.awt.AWTEventMulticaster.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent( Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(U nknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unkno wn Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(U nknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

it has happened every time for the last three weeks on three seperate projects

the backs up do not even work.