Oops, my URL for my image did not show up. Here it is.

http://image.inkfrog.com/pix/mcjane/00000001.JPG

It is Geranium robertianum, aka Herb Robert, a wild annual species that is considered a noxious weed in many parts of the country. Although quite hardy, often growing and blooming throughout winter in my climate, I wouldn't go to the effort of overwintering it. It self sows prolifically.

As Trevor Bath notes: "It often appears in the garden uninvited, looking delightful in the chink of a shady wall or paving. Turn your back and it will soon cover the whole garden, bedding itself comfortably in your most treasured small plants - you have been warned!"