format and reinstallation of xp in dual OS with vista

I am having both XP and vista in c and d drive respectively. due to some problem with xp i need to format and reinstal it. since m told its kind of tricky thing to do, as it would lead to loss of vista, which i dont want to do. anything by which i can do that.

or is it possible to just repair the XP without uninstalling it and how to do that as the bootable cd is not reading at the time of start up with normal window of "which of the os to load" asking