Unfortunately the solution is not a great one.
I had a similar problem on my MacBook a while ago, and the problem is actually due to file fragmentation, however no defrag tool could actually solve the problem. There are various solutions on Google indicating that a defrag does work, but I had no success with any of them.
The problem is related to how the files are spread on the drive, and BootCamp requires a continues 10GB or more of free space on the drive to be able to create the required partition.
The only way I got around the problem eventually was to format and reinstall the drive.
My honest advice is to look at using something like VirtualBox which is free, and create a virtual image for Windows. Depending on what you need to do, you can get away with a relatively small image. Windows 7 can run quite happily with only a 1GB of RAM if you don't need Aero. You can also look at paid for options like Parallels or VMWare Fusion
Alternatively, you could look at installing Windows on a secondary drive via USB. The speed won't be great, but if again, depending on what you need it for, it might be a sufficient solution.