May i know what is the way to run dropbox virtually without the need of having dropbox files store in my computer harddisk.
Dropbox is just a regular folder on your drive like any other, taking up disk space. It's meant to sync files placed in that folder with your account online. It is not a cloud-based folder where the content only exists online, although the Smart Sync feature will let you set individual files or folders as Online-only.
If you don't have access to the Smart Sync feature (i.e. you're on a free Basic account) then you can use Selective Sync to remove selected folders from your local drive. Once a folder is unchecked in Selective Sync, you'll use the Dropbox website to access it.