Definitely keep things simple and use import and export. For a start, the internal format used by Scrivener (inside the .scriv file) is .rtfd. Windows doesn’t recognise RTFD. It will show RTFD files as folders inside which you will find the text and any attachments separately. More importantly, though, whenever you edit a text file inside Scrivener, it saves the modified date and also saves a plain text representation of that text which is used for the search feature and for Spotlight. So, if you edited any files inside the .scriv package outside of Scrivener, search or Spotlight wouldn’t find those files.
So, I recommend that you export files you want to edit from Scrivener as RTF and then open them and edit them on Windows. You can save them and re-import them into Scrivener, deleting the originals.
Oh, and be sure not to use too many advanced formatting features in the RTF file. Annotations and footnotes should survive the process, as should most images (but not all) - at least as of the latest beta. But even then it’s not perfect…
I’d recommend getting a copy of RoughDraft (free) on your notebook, if it is still available. It is a nice little RTF editor, especially for those of us accustomed to easily working in multiple files. Notepad (or at least the version of Notepad I remember), only edits plain text files. At the very least, you’ll probably want to use WordPad, which is roughly equivalent to TextEdit.app, except in a Microsoft kind of way.
I still think you are going to want an RTF editor, unless you intend to lose all of your formatting and highlights and export as plain-text from Scrivener. Notepad++ is an excellent text editor, but like TextMate, it will open up an RTF file like a plain-text file, all of the codes visible. Something like RoughDraft will actually show bold as bold, and it also has a lot of tools for writers as opposed to programmers. But for your web site, definitely Notepad++ is better. So get both; they are both free, and are designed for completely different purposes.