with something similar to this for quite some time after I saw a statement in one of the other threads indicating that it was only feasible to create a dual boot system with LINUX/XP if XP occupied the FIRST partition on the hard drive (if XP and LINUX were sharing a drive). I had some trouble believing that that would be the case, but it proved to be more difficult than I had anticipated
I realize this is not exactly what your situation is, but what I discovered I think may shed a little light on your particular difficulties. As far as your existing XP installation goes, I'm not sure that I can give you an answer as far as how to successfully integrate that with whatever else it is that you are using without a more detailed forensic description of what exactly is on each one of your devices. However, I did find a way to make a NEW installation of XP pro work very nicely right alongside Mandrake 10.1 on the same drive.
If you are really serious about doing this, I would say to shelve your XP drive for the time being (that's nice that it still works), or copy all of your important files off of it and be willing to START OVER. That being said, it would be best if you could start with a system that has drives that have been completely WIPED of any OS to insure that you get a clean start. If you have extra equipment around, I would suggest using that FIRST for a couple of trial runs, because if your experience here is anything like mine was, you're NOT going to get this right the first time.
The only way that I could get XP to work successfully even AFTER modifying the MBR that it writes was to split the installation. The XP pro disc I have does its install in two basic parts (hopefully you have an install disc that behaves similarly to the way mine does)
1. install disc copies all the files it needs to a partition that it just let you create on your cleanly wiped HD, then reboots
BEFORE it has a chance to do this (but AFTER all of the files have been copied) yank the XP disc and put whatever bootloader BACK INTO the MBR (where incidentally XP just put some of its own stuff)- this was easy for me to do because I installed XP right after mandrake 10.1, so all I had to do was put LILO back where it was initially, the MBR (I know that isn't what you had, but this worked for me. With a little experimentation, I am sure you can get this to work in whatever config you happen to have)
2. Then, after you have verified that your bootloader still has the ability to give you access to whatever other OS you are wanting to use, use it to boot into XP (you should get a bluescreen or something that will inform you that Setup is resuming) and allow the installation to finish.
If you do this in this order, XP seems to be more tolerant of the fact that it is running on a system that does NOT have its proprietary MBR. Good luck. Realize that when you do this you are really gambling with the stability of your windows installation, XP can be really fussy about this. Don't be surprised if you end up having to go back and copy all your important files off that partition.