Okay well i am guessing you dont have a valid Vista install anymore right?
Have you used this Disc for XP before? Did you do a quick format or a full format when you went to install? Or did you not choose to format so that you can keep your info?
If you didnt format you will have to. It will have to be a full format. If you have Vista just right click on the drive you want to install on and go under Tools. You will see check disk there.