Windows Explorer in Windows 7 will remember your previous file searches. When typing in a new search, then it will perform auto-suggest using the previous searches.

It is possible to clear a previous file search by hovering over the suggestion in the search-box, and pressing the delete-key.

Turn off the search history feature by setting this DWORD value:

[HKEY_CURRENT_USER \Software \Policies \Microsoft \Windows \Explorer]
DisableSearchBoxSuggestions = 1

This should match the group policy "Turn off display of recent search entries in the Windows Explorer search box" (User Configuration | Administrative Templates | Windows Components | Windows Explorer)

Clear all cached search history entries by deleting the WordWheelQuery branch:

[HKEY_CURRENT_USER \Software \Microsoft \Windows \CurrentVersion \Explorer \WordWheelQuery]

Credits WinHelpOnline