I just installed Karmac Koala (Ubuntu) onto the same HDD as Windows XP, but Windows XP is not showing up in the grub boot loader. Windows XP was detected during the installation of Ubuntu, and I opted to use grub as the boot loader, so I don't know why this happened.
I don't know much about linux so if one could toss be a bone...
I'm actually going to read the Ubuntu Pocket Book soon, to get me started...