I think you can read all your Windows files in Ubuntu. You just didn't know how. Ubuntu doesn't allow a root login to the GUI but if you change to a root user in terminal by
you can see all the XP files.
In Linux every file has an ownership. In the name of security Ubuntu treats you as a normal user, knows the XP files belong to another user (possibly Admin) and denies you even from looking at them. Changing to a root user entitles you to become the Admin and so free access to all Windows files.
You should do a backup with an external USB disk now.
Your partitions are safe because standard Ubuntu does not permit you to write on NTFS partitions. You need to load ntfs-3g program before you can change or write a ntfs partition.
Depending how you have arranged Grub in Ubuntu, normally if you get a Grub prompt by hitting the "c" key then you can boot up your XP manually. The instructions in Grub prompt is
locate the first partition that has type 7 as it is a ntfs partition most likely your XP system partition. Take note of the partition number which always starts from 0. Say this partition is 0 then xp can be fired up by commnads
Code:
root (hd0,0)
chainloader +1
boot
The first line tells Grub the root partition you want to boot is the 1st partition in the 1st disk, as Grub counts from 0. The (hd0,0) is just an example. Adjust to suit your case. (hd0) means the 1st disk. The second line tells Grub to surrender the control to the XP's boot loader and that is all. Grub will passes the control over and buggers off.
If you want to boot XP without Ubuntu you can always retore XP's MBR back by booting up a Dos floppy or the FreeDos to do a "fdisk /mbr" or just get a win2k or any other XP installation CD to "fixmbr". More details in the last link in my signature.
I don't call this a problem, let alone a big one. Just a few basic knowledge of the two systems.