I have done this before, and I just recently deleted Windows because I needed more space on my Mac. Now that I have everything situated, I want to get Windows back, but I keep getting this message.

I'm not really sure why my post was deleted...
Have you tried creating a new partition in Disk Utility, then removing it and retry Boot Camp Assistant? If I remember correctly, partitioning in DU does in fact defragment the disk (to some extent).
How would I go about doing that?
cmd+space, enter Disk Utility, hit enter, choose your hard drive, click on the button that says Partition, click on the little plus sign underneath to add a new partition.
As I said in the other thread where you posted this, no, partitioning does not defrag a drive. However, there may be times when you need to defrag in order to partition a drive.I'm not really sure why my post was deleted...
Have you tried creating a new partition in Disk Utility, then removing it and retry Boot Camp Assistant? If I remember correctly, partitioning in DU does in fact defragment the disk (to some extent).