centos/xp DUAL BOOT different HDD
 

I have a windows xp installed on a hdd on a particular machine and I also have centos 5 installed on another hard drive.
I want to add the hard drive to the xp machine switch the boot sequence to the centos HDD and now I want be able to load windows from GRUB.

It sounds like your taking the Centos drive from one computer and adding it to the one with Windows. Chances are the hardware structure between the two computers are quite different, some operating systems don't adapt easily to this sort of change.
You can however, plug it in and set the bios (setup) of the computer to boot the second (new) drive and see what happens.
If it's happy, you can proceed to install grub to either drive from command line in the running Centos, normally to the drive set to boot first in the bios, which should be the Windows drive to keep things simple, and it will add Windows to the menu, reboot, switch back to Windows drive in bios as first boot drive and continue booting.

Thank you for the quit response.

However I've already tryed that method, I even went on to add these lines to grub.

title Windows

rootnoverify (hd1,0)
makeactive
chainloader +1
boot

being that

windows is sdb1
and centos is
sda0
sda1

and when it tries to load it hangs.

Is not possible for you to plug the HDS in that way that
Windows become SDA
and centos SDB
Because linux do not have any problem to start from any drive.
I do not know if windows can start from sdb
In that case widows is (hd0,0)

It is possible to boot windows from sdb. You need to add a couple of commands so that windows thinks its actually booting from sda.

title Windows
map (hd0) (hd1)
map (hd1) (hd0)
rootnoverify (hd1,0)
chainloader +1
makeactive