Search assistant pops up at start

Whenever I have to reboot or restart Windows XP (home edition), the 'Search
Asstistant window' appears on the desktop with the left pane (with the dog)
flickering.
I have formatted the hard disk and installed Windows XP (home edition) again
and made a new MBR without contacting Internet, so I assume no spyware of
virus can be the problem. With the task panel I can end the program, but as
soon as I move
with the mouse pointer to the start button, the search window appears again.
Only after opening Windows explorer the search assistant does not return.
Does anyone have a clue as to how to stop this from appearing ??