Comment 2 for bug 1030631

It didn't download the preinstalled image properly. You should have 489MB not 362MB (380523926 byes). The installer should probably recognize this instead of trying to proceed.

So as a workaround, you could try again (probably some network error that stopped it fully downloading), or manually download the target file (see here for instructions how to use it: http://askubuntu.com/a/143478/14916 ), or download the desktop cd ISO instead (save in same folder as wubi.exe before running).