So what your trying to do is use an iMacs display as an external display for the Air?
As far as I remember the latest iMacs can now use the display as an external display for another computer so it should work. (As long as the iMac is off anyway). I would imagine it would be up to Windows or Boot Camp if it will support it under Windows though.
In the upper Finder menu on the iMac, click on Help and type in "Using iMac as External display" (or something similar) and it should tell you how to do it or what's required? Make sure you're connected to the internet as some of the help information is accessed from Apples site.
Found this post, might help?