OK, make a good backup of your system (I'd hate to trash it for you)
First you need to put your user in the group 'disk
', if you don't do this the vmdk you create will only be useable by root
enter usermod -a -G disk <your user name>
log out and then back in
as your normal user
you now have some options
Assuming your grub is written to to /dev/sda1 create your new vmdk using VBoxManage internalcommands createrawvmdk -filename /mnt/data/VBOX/HardDisks/WindowsXP.vmdk -rawdisk /dev/sda -partitions 1,8 -relative -register
your new machine will use the current grub menu for your system and you will have to select your Windows installation from the list. be very careful with this as the chances of corrupting your system by selecting PCLinuxOS by mistake are pretty high
You could also use VBoxManage internalcommands createrawvmdk -filename /mnt/data/VBOX/HardDisks/WindowsXP.vmdk -rawdisk /dev/sda -partitions 8 -relative -register
When you boot this machine it will fail with a grub error, you should be able to fix this using your windows CD (or an image of it) and booting this first. Press the "R" key during the setup in order to start the Recovery Console. Select your Windows XP installation from the list and enter the administrator password. At the input prompt, enter the command "FIXMBR" and confirm the query with "y". The MBR will be rewritten and GRUB will be uninstalled. Press "exit" to reboot the computer. This should only have an effect on the virtual machine (you did back up didn't you
The final option is to create a file containing the mbr of another system (e.g. winxp.mbr) and reference this when you create the disk VBoxManage internalcommands createrawvmdk -filename /mnt/data/VBOX/HardDisks/WindowsXP.vmdk -rawdisk /dev/sda -partitions 8 -relative -mbr winxp.mbr -register
Again, this should only affect your virtual machine.
Good luck and be careful.