Windows Vista automatically defragments your hard drive according to a set schedule. While this option is fine for those that regularly forget to do the basic housekeeping such as defragmenting the hard drive, it can be a little awkward if your PC doesn't happen to be on when the defragmentation schedule kicks in.
To disable automatic defragmentation proceed as follows:
1/ Click the Start button
2/ From the Start Menu, Click Control Panel
3/ In Control Panel, Click the System And maintenance option
4/ In the System And Maintenance window, Click the Performance Information and Tools option
5/ In the Performance Information and Tools window, click the Advanced Tools option situated on the menu to the left of the main window
6/ In the Advanced Tools Window, Click on the Open Disk Defragmenter Option
7/ The User Account Control will now appear asking for permission to complete this task
8/ In the Disk Defragmeter window remove the tick from the Run On A Schedule (Recommended) option and then Click OK
9/ Automatic defragmentation has now been disabled