Got this from stackoverflow site... I didn't bother trying it or anything...

Code:
int hours = (int)hoursDecimal;
decimal minutesDecimal = ((hoursDecimal - hours) * 60);
int minutes = (int)minutesDecimal;
int seconds = (int)((minutesDecimal - minutes) * 60);
Once you have that, I would just format it using
Code:
String.Format("{0}:{1}.{2}", hours, minutes, seconds);