I am installing YUN on Windows XP as per istructions to select port the only visible option in the dropdpwn is "Com1" I have YUN connected to PC with USB A to A cable. 0.
Highly unlikely that you are seeing the YUn on COM 1.
Which IDE are you using and have you updated the board defs etc.
TOOLS BOARD BOARD MANAGER
Select Arduino AVR and see which version you have. Update if possible but be prepared to roll back if you have any issues as XP can be fussy.