First of all, it's 10.04 (as in "201004"), **not** 10.40. The release number corresponds to the year (10) and the month (04) that the version was released.
Secondly, the Ubuntu community was thinking of removing Wubi (the Windows Ubuntu Installer) from 10.04, as it was causing plenty of problems.
My advice: Try the 10.10 beta, boot from the CD, start Ubiquity (the Live CD installer), and when you get to the disk partitioning stage, select "Install side by side, choosing between them each startup". The one thing about 10.10 is that it has a very user-friendly partitioner that essentially looks very much Mac OS X's Boot Camp Assistant: It has a slider that allows you to allocate the disk space with one click and swipe of the mouse. Wubi is by far the most buggy way to install Ubuntu there is.