Looking at your screenshot above, the plugin you have installed should be able to do it. In order to keep your posts & media and comments you would need to leave out the posts, postmeta and comments options. The one related to postmeta will contain everything to do with who posted it, post id, media included etc, although I would advocate the option I set out next, as it is less likely to mess everything up.
A separate option is install a fresh copy of WordPress, and then export your current blog into your new one. This will mean you can keep everything you need, but you get a fresh database etc to use.
The options are under Tools --> Export to export the blog (you can pick what to export) and Tools --> Import to bring everything into your new WordPress install (This does require a plugin, but it is mentioned on the import page).