Angus,
does the same happen, when you are launching iTunes from a different user account? For example as a Guest user? If iTunes is o.k. in a diffrent user account, your preferences might be broken.
Then remove your iTunes Prefrences file com.apple.iTunes.plist from the Preferences folder in your User Library.
The user library is hidden by default in Lion and Mt. Lion. To reveal it, use the Finder's "Go" menu. Select "Go" in the Finder's main menu bar and hold down the alt/option key, until you see the "Library" in the drop-down menu. Double click it to open a Finder window, so you can browse the files inside.
But be aware, that after removing the user preferences you will have to reset all settings that have been added using the Preferences panel. So take a note of all settings you entered there, before you remove the file.
Regards
Léonie