Window's XP program to carry out disk defragmentation is helpfully called Disk Defragmenter.
You can access t these tools by right clicking in Windows explorer the drive you want to maintain and selecting Properties | Tools tab. If you add, delete and modify a large amount of files then you should defragment your computer monthly. As most of these files will be data files this is another reason for splitting your data and programs on different drives/partitions so that the time taken to 'defrag' is reduced as the programs will not be altered often. The program drive should be analysed after each upgrade of software to check if a 'defrag' is required. Windows XP will tell you if it is required.