WubiGuide says to use the older versions found at Sourceforge.

This relies upon a distribution (ISO) located on the http://de.archive.ubuntu.com/ubuntu-releases/ server. However, they only have the 8.x, 10.x and 11.x releases, and Wubi fails to install Ubuntu with a "404-file not found" error.

However, if you place an ISO distribution of Ubuntu 9.04 on any of the target computer's drives (including a memory stick), Wubi will find it and use it for the installation. I got the 9.04 distribution from http://old-releases.ubuntu.com/releases/9.04/.

Why install 9.04? It is the version supported by Dummynet.