If you use a NTFS file system, you will not be able to modify the XP Partition's icon by the "Get Info" procedure. Follow this instructions to see how you can change your XP Partition's icon
1.- Format a USB stick with a FAT file system (you can use any volume really, but it needs to be FAT).
2.- In Mac OS X, set a custom volume icon using the Finder's "More info" window or any tool of your choice.
3.- Open a Terminal window, enter
cd /Volumes
ls -la
You'll see a file listed that has the same name as the FAT volume, but with a "._" prepended. For example, "._USBSTICK".
4.-Copy that file to the equivalent name for the Windows XP NTFS partition by typing in the terminal
cp ._USBSTICK ._WinXP
5.- Transfer the ".VolumeIcon.icns" file created on the FAT file system to the root of the NTFS partition, using Windows.
6.- Open a Command Prompt and Enter in "attrib +H +S .VolumeIcon.icns" from the root of your NTFS partition. This will make the file a Hidden, System file, which hides it from curious passers by and even if you have the show hidden and systems files it will not show up in your NTFS partition.
Congratulations! Now your XP partition has a new icon