To clear the Past Items list, perform the following steps:
1. Delete the IconStreams and PastIconsStream values from the following registry key:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\TrayNotify
2. Open Task Manager, click the Processes tab, click Explorer.exe, and then click End Process.
3. In Task Manager, click File, click New Task, type explorer, and then click OK.
from
http://support.microsoft.com/default.aspx?kbid=283084