All you need to do is to 'shred' (rather than simply 'delete' ) your documents, etc, using this:
http://www.fileshredder.org/
You should also delete your browser's cookies and stored passwords (and possibly your bookmarks as well).
If you used a mail client on your netbook (rather than just webmail), delete all of your emails and the actual account(s) as well.
Uninstall any software which accesses subscription-based services.
That's far simpler than returning the netbook to its factory settings.