If you have items in Past Items that are have long ago been removed from the system and want to remove them from the list, use the following procedure.

Click Start > Run > Regedit

Navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\TrayNotify

Delete the IconStreams and PastIconsStream values.

Open Task Manager (ctrl-alt-del), click Processes tab, click Explorer.exe, then click End Process.

While in Task Manager, goto File > New Task > type explorer, then click OK

This will reset your icons and populate the bar with your currently running apps that make use of the tollbar area.

Be Blessed!
Chris