This is from Microsoft's Knowledge Base...
WARNING : Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.
For information about how to edit the registry, view the "Changing Keys and Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT or Windows 2000, you should also update your Emergency Repair Disk (ERD).
To clear the Past Items list, perform the following steps:
Delete the IconStreams and PastIconsStream values from the following registry key:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\TrayNotify
Open Task Manager, click the Processes tab, click Explorer.exe , and then click End Process .
In Task Manager, click File , click New Task , type explorer , and then click OK .