Linux (Ubuntu) question, dual booting?

Hello, I’ve been playing around with Ubuntu for a bit from the live cd and want to dual boot it with XP. If I use the guided option on the installer it says there’s not enough space for a partition or something. If I go into manual it says there’s 8MB free space and the rest (39999MB) is ‘unknown’. I know there is at least 25 GB free space on the hard drive. Any ideas for how to solve this? (Ubuntu version is Hardy Heron). Thanks