Indexing Service

• The indexing service in Windows maintains an inventory of the most commonly accessed files on the hard drive. This allows Windows to display the location of many files almost instantly when you search for them. To keep the index updated, Windows periodically scans the indexed locations on the hard drive for new files, causing hard drive activity. Click the "Indexing Options" icon on the Control Panel in Windows 7 to modify your computer's indexing settings.
  
  

Windows Updates

• Microsoft recommends that you configure the Windows Update utility to update the computer automatically. Using this setting, the computer periodically checks for new updates and downloads critical security updates automatically. You will see your computer's hard drive activity light flash when the computer is checking for, downloading and installing updates.
  
  

Scheduled Tasks

• You can use the Task Scheduler in Windows to automatically run applications such as the Disk Defragmenter during times when you do not use the computer. If you have created any scheduled tasks, they may cause hard drive activity when the computer is idle -- particularly the Disk Defragmenter application, which causes constant hard drive activity until the defragmentation task is finished.
  
  

Background Programs

• The background programs on your computer may have components or features that activate automatically without your input. These programs might include an anti-virus utility that downloads new virus definitions or performs periodic scans, software that updates itself automatically or file-sharing programs. Press "Ctrl-Shift-Esc" to open the Windows Task Manager and see what applications are running.