Okay, it seems to be an issue with updating to grub 2 specifically within a wubi installation.
What appears to be a solution is here
http://www.omaregan.com/?p=583Actually an updated (and easier fix) is here
http://www.omaregan.com/?p=608Alternatively, if you have now decided you are happy with kubuntu it might be time to consider a permanent home for it which would mean
1. Uninstalling it from within windows (after a backup of course) and then
2. Installing it side by side i.e as a formal dual boot.
If you decide to take the second course, come back here for guidance on the best way to do it.