Unable to install equinox p2 plugins for Eclipse Ganymede

I have been trying to install "build utility feature for equinox p2 plugins", on which Spring IDE Eclipse plugin and M2Eclipse plugin have a dependency. But I'm constantly getting an "Invalid zip file format" error:

An error occurred while collecting items to be installed
Error closing the output stream for master-equinox-p2/org.eclipse.update.feature/1.0.0.v20080506-4--8Mc44yANsYbyiqu-z-uDo0 on repository file:/C:/eclipse-3.4-ganymede/.
Error unzipping C:\DOCUME~1\Alex\LOCALS~1\Temp\master-equinox-p2_1.0.0.v20080506-4--8Mc44yANsYbyiqu-z-uDo044914.jar: Invalid zip file format

It seems to be unable to download a good copy of master-equinox-p2 JAR file. I have cleaned up all the temporary internet files and tried to install on a different machine but still no luck. I suspect that the mirror site is corrupted, probably with a bad signature or something. But it doesn't seem to allow me to choose mirror site in Ganymede any more. Urrghh!