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