What you want is not easy or automatic. One way is to turn off iTunes maintaining and organizing your media and doing it yourself. Are you super organized? Another is to hold down the option key while dragging media to iTunes when wanting to keep it in the secondary location (the main one stays the locatinn in iTunes preferences). You have to remember to do this each time.
Remember your Windows drive may need reformatting to use with a Mac unless you buy special software. This will erase all data. Windows uses NTFS format which Macs can only read but not write.
This is starting to get into complexeties. iTunes has two main components. One is your library files which store all the information about file location. Another is the media files. Unless you are permanently connected to this Time Capsule (you say you have a Macbook which means you may not always be) you will have to have the library files on the internal drive or iTunes won't run. You then have media split up. By the way, if you start iTunes and the external is not connected, all those files will show with broken links. They will fix again when you restart iTunes with the drive attached (you have to restart iTunes), but it can be distracting.
What are the iTunes library files? - http://support.apple.com/kb/HT1660
More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management
What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html
Where are my iTunes files located? - http://support.apple.com/kb/ht1391
If you main need for the external storage is bulky movie files you can consider starting a second separate library and just keep movies there. I don't see the need to have my movies and music in one library.
Here's some more tips but until you review the above and maybe dcide on a different track I won't post detailed steps.
iTunes: How to move [or copy] your music [library] to a new computer [or another drive] - http://support.apple.com/kb/HT4527 - a somewhat bewildering and not always easily understandable set of options.
Quick answer if you use iTunes' default preferences settings: Copy the entire iTunes folder (and in doing so all its subfolders and files) intact to the other drive. Open iTunes and immediately hold down the Option (alt) key (shift on Windows), then guide it to the new location of the iTunes Library.itl file in the moved iTunes folder.
For the record there's this reference for iTunes 11 but it really doesn't strike me as having the specifics you need. iTunes 11 for Mac: Move your library to another computer - http://support.apple.com/kb/PH12168