Dropbox is a File/Photo sharing service (Example included) which lets folks like me post photos to places like this.
AVAST has a feature called SCAN for outdated software (This is under the SCAN menu on the user Interface)
It checks with the Dropbox, Firefox, Thundrbird, OpenOffice, Whatever software you have, web sits to see if there are updates and when it finds one it attempts to download and install. This is NOT bundled crapware as with so many other programs.
Dropbox was mostlikely installed on your computer by the manufacturer (Was on some of mine as I recall)
Enjoy the sunrise: