Ah, ill go into it on a basic level.
Microsoft are wankers in a sense. Its either their way or the highway for many things, including bootloaders.
Theirs is written into the MBR with no support to boot other OS's unless you want to get a dos boot thing which is a big pain. Grub is also written into the MBr which kind of sucks as they dont get along that well.
There is a way to get around it, which is installing the windows bootloader, then installing grub and having grub chainload XP 64. It should automatically create a boot option for xp64.
Now for getting it to work
When you say "even after repair" are you referring to repairing XP or ubuntu. To get that to boot i would install Xp64 and then install kubuntu. To avoid reinstalling them just put in ur XP64 cd, enter the recovery console and type fixmbr.
After doing that whack in your kubuntu live cd and then get back to me if this doesnt work (this is all from memory!)
sudo grub-install /dev/*** <--- drive name, if you need help determining it just ask.
After running that grub should be installed into the MBR alongside the XP loader, and then it should have all the options for the Kubuntu system as well as Xp.
If you want XP to be the first option you can open /boot/grub/grub.conf and change it to be default.
And if you get tired of linux and want to wipe it all, just delete your linux partitions, load the windows xp64 cd, enter the recovery console and type fixmbr
Anyways get back to me as to how far my guide goes before it stops working
