1. Click on the Dropbox icon in the System Tray at the bottom right of your screen.
2. Click on the configuration wheel of the window that pops up.
3. Select 'Preferences…'
4. In Preferences, select the 'Account' tab.
5. Click 'Selective Sync…'
6. Uncheck any non-critical folders. The implication is that these folders will not be downloaded to your computer, but will remain in Dropbox online.
7. To view your files in Dropbox online, click on the Dropbox icon in the System Tray and then select 'Dropbox.com'.