When developing for Sharepoint, in order to deploy you need to run Visual Studio in Administrator mode. To make this the default behaivor:
- Right click on the shortcut (works from the start menu too)
- Switch to the compatibility tab
- Click Change settings for all users
Run this program as an administrator
And you are done.
EDIT: In order to launch VS as Admin when opening solutions in Windows Explorer, please see Wimbo's comment.