Boot help!! what am I missing???

I'm stuck, don't want to wipe this drive unless I absolutely have to, the kid was running some PC tuneup program and messed up his xp pro boot sector. I have tried a xp cd, ran chkdsk /r, fixboot and fixmbr. I did bootcfg /rebuild and all that did was create another win xp entry in the startup, but nothing works, it goes to the safe mode, last known good menus but none of them work, they all freeze up. I have an UBCD4 CD and have backup up some of his data with it to an external drive, but he wants 100 GB saved! My external is only 80! How can I get this starting again? Can someone tell me how to build the boot files? Can I copy some system files boot.ini or ntldr or something? I afraid to do a windows repair install, he doesn't have a cd key sticker to get his xp validated, the system was custom built and they never supplied him with one, help!