There are a number of NT disk defraggers on the market, including Winternals Defrag Manager. These tools are useful for performing a general defragmentation of disks, but while most files are defragmented on drives processed by these utilities, some files may not be.
In addition, it is difficult to ensure that particular files that are frequently used are defragmented - they may remain fragmented for reasons that are specific to the defragmentation algorithms used by the defragging product that has been applied.
Finally, even if all files have been defragmented, subsequent changes to critical files could cause them to become fragmented. Only by running an entire defrag operation can one hope that they might be defragmented again.
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.