Sysinternals provides the Contig utility, which you can use to defragment individual files or folders.
You can download the tool at the following website:
To defragment a single file, use this format:
To analyze the fragmentation status, add the -a switch as in the following example:
X: emp>contig -a en_whatchmacallit.iso
Alternatively, you can use wild cards in addition to the -s switch to search through the path given and any child folders and perform degragmentation on files matching the given file name criteria. The Contig tool is a great utility to keep important files defragmented without having to worry about the entire disk.