HDD can't be partitioned "files can't be moved"

I'm trying to install Windows 7 on my Macbook Aluminum from late 2008 and when I go to partition my drive it says that the files can't be moved. I have over a hundred twenty gigabytes free so I wanted a 70GB partition for Windows. I've tried in Boot Camp and Disk Utility and neither works. It keeps stopping and giving me the error messages that the file system cannot be moved. I've looked at the forums and all I've found is that the problem comes from having too little space to partition and that is clearly not my problem. Can anyone please help me? Thanks!