I'm using several cloude storage services, like Dropbox. Each have windows app for its synchronization, and this app actually can read anything from my disc, not just files from its 'synchronized' folder. I want to make sure that this apps never access any other folder except sync folder. Is there a way to do so (using VM is not an option - too much resources required).
Create a new user.
Please note that you might have to allow access to other folders where Dropbox keeps its cache files and configurations.
You can use a user-mode sandbox such as Sandboxie. Sandboxie intercepts API calls and only allows an application access to specific resources you define.
NOTE: I'm not sure how dropbox is implemented. If it runs as a service it may not work. It's still worth a try though.