I have been using Artha now for a couple of years, and it is more than just a standard thesaurus.
It is both a dictionary as it provides the meaning(s) of entered queries, and a thesaurus because it provides synonyms, antonyms, derivatives plus a number of other relationships to your query. Even better, it allows you to follow paths through those relationships by double-clicking any word in the interface and has navigation buttons to delve backwards and forwards through your search history.
It is 100% offline because it bundles the WordNet database and as such doesn’t need to connect to the internet, it is open source and thus is completely free, it supports WindowsXP and up, and is 32-bit so will run fine on both 32- and 64-bit systems.
The only thing it doesn’t have that you requested is Notepad++ integration.
For the sake of disclosure: I am the packager of a portable version of Artha, but am in no way affiliated with Artha itself. I don’t and can’t earn anything from any download of Artha or any derivatives of it.
