When you right click on a file or folder, you can move it to the Dropbox folder
Add Dropbox to the Send To menu in Windows 7, XP and Vista Picture 4
If you already have a shared folder in the Dropbox folder, you can also add them to the Send To menu along with the same method. As in the example, we're adding a shared Dropbox folder.
Add Dropbox to the Send To menu in Windows 7, XP and Vista Picture 5
Add Dropbox to Send To in XP operating system
In Windows XP, go to Control Panel > Folder Options > Show hidden files and folders.
Add Dropbox to the Send To menu in Windows 7, XP and Vista Picture 6
Go to C: Documents and Settings [User Name] SendTo (User Name is the name of your computer) and create a shortcut to the Dropbox folder.
Add Dropbox to the Send To menu in Windows 7, XP and Vista Picture 7
Now you have Dropbox folder added to the Send To section in Context Menu.
Add Dropbox to the Send To menu in Windows 7, XP and Vista Picture 8
If you use Send To in the Context menu, it's easier to add files and folders to the Dropbox folder.
( Source : Howtogeek)