Forgot my old microsoft id used in windows 8 pc,help to reset it.

i forgot my  old microsoft id and password for ,y windows 8 pc and in skype they are asking my microsoft credentials,and i am not able to install from dell store since i forgot the id.is there any way of getting it or how to change it to new credentials so that my problem can be solved?my dell pc service tag is JWP72X1