Alright peoples, here's my solution to this problem:
I had Parallels installed, my HD was almost full (2GB left) and I wanted to install XP via BootCamp. So, I deleted large files, and uninstalled Parallels. That gave me 18GB free space, but BootCamp still wouldn't work: 'some files can't be moved'...
Solution:
Get iDefrag, and take a look at your HD. In my case, a lot of scattered data was sitting around all over my HD, so it was impossible for BootCamp to create a 5GB partition at the end of the HD, although I had a total of 25GB free space (I deleted whatever I could). Since iDefrag won't work from within OS X (it can't move system files that are in use) you have to get Coriolis CDmaker (Comes with iDefrag if you buy it). This will let you burn a bootable CD with iDefrag on it. Restart from CD, let iDefrag run a Full Defrag (took 8 hours on my 80GB drive) and then you have a beautifully defragmented HD with all the free space at the end of the drive. This will allow BootCamp to set up the new partition in just seconds....
😀
And I'm not gonna tell you where to get the programs, use your imagination 😉