I recently installed 11.04 from an installation disk (overwriting Windows in the process). I know 11.04 is no longer supported but I had no problems subsequently upgrading it to 12.04 (via 11.10) a couple of months ago on another device.

This time though, things are different. I can't upgrade through update manager because Ubuntu then tells me I have no internet connection, which is (obviously incorrect).

I have tried to circumvent the problem by downloading the 12:04 iso from ubuntu.com directly but now I'm troubled by something else.

The download is succesfull but after mounting the iso I can't interact with it. When I try to access the Wubi it gives me the following message:

Archive:  /home/lars/.cache/.fr-7g75Fe/wubi.exe
[/home/lars/.cache/.fr-7g75Fe/wubi.exe]
  End-of-central-directory signature not found.  Either this file is not
  a zipfile, or it constitutes one disk of a multi-part archive.  In the
  latter case the central directory and zipfile comment will be found on
  the last disk(s) of this archive.
zipinfo:  cannot find zipfile directory in one of /home/lars/.cache/.fr-7g75Fe/wubi.exe or
          /home/lars/.cache/.fr-7g75Fe/wubi.exe.zip, and cannot find /home/lars/.cache/.fr-7g75Fe/wubi.exe.ZIP, period.

What am I doing wrong here?