Sounds like you have a virtual memory page file, or a hibernation file or similar stuck at the end of the disk. I would boot from a CD or other media (Recovery partition if on Lion), startup Disk Utility from there so you have access to the whole drive without the encumberence of actually running on the drive you are trying to play with, and rezising the partition from there to the desired size. This will stop you encountering any files that are "locked" which prevented the resize from taking place previously. Then you can boot back into it, and carry on with the Boot Camp Assistant.
Take a backup first, this applies regardless of what method you are using, if you are resizing your partitions, then a power cut could easily screw the lot. Another method would be to backup, recreate the partition smaller, and restore into it like the dialogue screenshot you provide says, but this is overkill and you should be able to do it without needing to complete all those steps, just have the backup handy as a safety net, and resize while booted from another OS instance.