At point c the voltage should be 0 because it's grounded. I'm assuming it goes counter-clockwise. So would it be 5V at point B?
Adding up the voltages is:
5 + 4I - 15 + 1I = 0
I = 2
So then at point A it would be
4*2 + 5 = 13
At point D it would be
15 - 13 = 2
and then...
http://img519.imageshack.us/img519/3416/32ex28.jpg
What is the voltage at points a,b,c,d? I don't know how this works with Kirchoff's loop law. Any help?