Hello and welcome to my Use Tip
"This disc can not be partitioned because it is impossible to move some of its files."
So you saw this in BootCamp and know wondering what to do.
Some have even tried iDefrag or other defragging software and it didn't work, it's because they only defrag and not shift data up nearly as well as it can be.
Hard drives write data from the top down, if you delete data it doesn't magically shift upwards so there is all the remaining OS X data on the lower part where BootCamp needs to go.
There is a minimum size for the Windows 7 BootCamp partition, and because OS X data is on the area where Bootcamp needs to go, it can't be created.
The only solution is to use a blank powered external drive and (free, donations accepted) Carbon Copy Cloner and clone your OS X partition to the external drive (default settings fine), then reboot holding the option key down (Startup Manager) so you can select this external drive clone to boot from.
Once you have marveled at the coolness factor of a bootable clone, you have to erase the internal OS X partition (while booted from the clone) using the Disk Utility > Zero erase (one step below max security on the right in 10.7) and once that's finished, to use Carbon Copy Cloner to clone the clone to the internal OS X partition.
click on picture for a larger view
Once you do that you can boot the computer normally and then easily create the BootCamp partition free of data and of any bad sectors on the drive which the Zero Erase function assisted with.
It's a pain and takes a long time to complete, but it's the only way to shift ALL and ANY data upwards on the OS X partition to make room for BootCamp, which starts at the bottom and works upwards until it hits OS X data. Cloning software does special magic to ensure the OS X clone will boot.
One can of course choose to backup file and Erase and Install OS X fresh too, that will work, the cloning process avoids all the work.
Hope that explains your issue.
If it went over your head, perhaps you need to have a professional install BootCamp and perform this function for you.
Learn more about clones here, and about WinClone since your going to be using Bootcamp.
You might want to consider using Windows in a virtual machine program instead which Windows is basically just another file in OS X and thus a lot easier to manage than a direct boot in BootCamp.
See the pro's and con's of each here.
Performance Tip: This clone and reverse clone process also so happens to optimize your OS X partition, defrags and make your programs launch faster on hard drives, all in the process of having a copy in meanwhile for safety and backup reasons.