problem repairing windows xpsp2

I have 2 os on my pc. One is ubuntu and the other is windows xp .
Since my windows is failing to boot i tried repairing it . But when the pc reboots during repair it asks me to choose between the 2 os . And when i choose xp the same problem reappears giving me a blank screen with no further progress.
Please suggest me a solution to effectively repair my windows because i dont want to format my C drive again.