Kegerator is shocking me

Question

ive got a kegerater its a older unit. only has the 2 prung plug. (no ground) now when i plug it in i get a slight jolt when i touch the tower/metal. however the prior owner had a ground wire attached to the side then ran it to the socket. in my upstairs my outlet is not grounded so i cant do that. what other options do i have? why is it doing this shocking?