Like changing the oil in a car, defragmenting partitions is a requirement, not an option. On a heavily used system, you should be defragmenting your disks at least once per month to ensure optimal performance. Remember that files are stored on a disk in clusters. Over time, as files are added and deleted, these clusters can become rearranged on the disk, or stored in non-contiguous block of space.
A badly fragmented disk results in very poor performance, as the drive needs to rebuild the data stored in those clusters when you want to open a file. If your disk seems slow, chances are good that all it needs is a good defragmenting. Use the Disk Defragmenter included with Windows XP to accomplish this, understanding that 3rd party defragmentations tools are generally more robust and provide for better optimization. For those looking to automate, Windows XP includes a command line defrag utility, which could be used to schedule the process automatically.