Defrag a single file with Contig

Contig is a single-file defragmenter that attempts to make files contiguous on disk. Its perfect for quickly optimizing files that are continuously becoming fragmented, or that you want to ensure are in as few fragments as possible.

Another great utility from SysInternals

Link:
http://technet.microsoft.com/en-us/sysinternals/bb897428.aspx