Your suggested method wiuld be the best, as you have only recently installed the OS. But, you say you have the ISO? Burn it to a DVD and boot up from the DVD. In the early stages of the installatin, you will be offered repair options. Select this and "repair start up".
But this will still leave you in a position where you cannot boot XP. It will require a bit of work, and copying and transfer of XP boot files, should they be avaialble.