Olly

I wanted to reformat my GDs computer so I put the operating system Windows Vista Home Premium onto the D: drive. I then formatted the C: drive. Once C: drive was clean I put Vista back onto the C: drive. Now when I switch on it shows both installations before I can log on. I have tried to remove the operating system off of the D: drive but I have had no luck in finding a way to do this. I have tried to reformat and it will not let me. Any help would be appreciated.