http://pcsupport.about.com/od/operat...txprepair1.htm
I've been through similar what you are experiencing. The link above explains everything you need to do. Read it carefully.
The crux of it is this. Boot from your XP disk. When you get to the blue screen menus, you will be doing a repair of XP. But do not press R or select the second bullet, "recovery console" option (pic at slide 4 /10 from link above to see what I'm talking about).
You want to pick the top bullet, "To set up window xp now". Then at slide 5/10 say "yes" to the end user agreement. Then slide 6/10
you should see your C drive in the option box. If it is not there, then stop. Something is wrong.
My advice is free, so assume at your own risk
Addition:
- to boot from the disk, you need to go into the bios and set it up to boot from the DVD drive.
- after you have repaired XP, you need to go back in the bios and set it up to boot from C drive again.