This is a known bug in the treeview control. It has existed since 2005, and is known to occur for projects running under Vista with visual styles enabled.
The only real workaround is to create a wrapper class for the treeview control and discard the double click event.
Would include links to microsoft references of the bug, but can't put them here. Google bug id 358695. Might also google 092fe0a3-073c-46d6-8d4b-10ddf7329526