Instead of running a disk defragmenter on a single partition, it might be faster (and more optimal) to run a file defragmenter. Contig, from Sysinternals, is a a freeware that uses the native Windows NT defragmentation support to analyse and defrag specific files.
For instance, if you use a system-managed pagefile, your system performance may be slow if your pagefile is heavily fragmented. Hence, it would be wise to defrag the pagefile (normally hidden at C:\pagefile.sys). Defraging hiberfil.sys may also improve the time required to resume form hibernation. Hence, defraging heavily used files (which have high-tendencies to be the point weakest links), may generally improve system performance.
For those who prefer not to use the command line, PowerDefragmenter (combined with Contig), is the solution.