Windows Explorer shows the files/folders that are actually on your computer. I don't think it's possible to make it show files that are in your Dropbox account but haven't been downloaded to your computer.
You could sync all of your files to your computer and then disable syncing for the folders that you don't want to stay synced. Personally I would find that to be confusing, but you could do that if you wanted to.
You could also bookmark https://www.dropbox.com/home and open it whenever you need to check what's in your Dropbox account. That's not too far off from viewing the files in Windows Explorer.
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