The easiest way would be to install Windows, and then Ubuntu, but either way will work.
First, I would suggest shrinking the Linux partition to something closer to 10-15GB. you can always add partitions later for data, etc. When you start filling up your system with data files, then it is really a pain to re-size and move things around.
specifically--on a disk that size--I would start with:
Shared data: 50-100GB (The file system for shared data is a whole separate topic--easiest for starters is FAT32)
The rest empty (unpartitioned)
When you install XP it will take over the MBR (no choice). Thus you will have to re-install GRUB---you can do that from your Ubuntu LiveCD. Or--make a GRUB floppy from Ubuntu before installing Windows.
When you start the XP install, it should see the empty space on the disk and prompt you as to how to set up the Windows partition.
Follow the link below for more on booting and GRUB, including how to set up GRUB to boot XP.