BUT I CAN open Dropbox.com on same network on my iPad and iPhone.
Something on your computer is blocking access to Dropbox. Are you sure you're only running the built-in antivirus? There have been reports lately that BitDefender has been incorrectly blocking Dropbox.
Whichever antivirus you're using, temporarily disable it to see if you can connect to Dropbox. If you can, then you'll need to add an exclusion to your antivirus so it doesn't block Dropbox.