Why we have to manually start the disk defragmenter in Windows XP?.
You don't have to. You can make the computer automatically start it for you.
If you use JKDefrag or Mydefrag you can set defragmentation as your screensaver and it will start whenever your not busy. That solves all your problems!
Allows you to select individual files to defrag, can be configured to execute in the background, can be scheduled to run automatically at specific times.