unexpected error excel 2003

I have a new workstation with Windows XP PRO and a fresh load of Office SBE
2003. When my user enters a number in a cell, goes to the next cell and does
an A2+1 formula to add 1 to the previous number - then, copies and pastes
that formula down several cells in the row....... if he holds down CTRL and
presses his down arrow (to move the curser to the last cell), he gets the
Unexpected Error wanting him to report to MS. Any idea's as to why he can't
Ctrl down arrow? This happens regardless of which directtion he tries to
move the curser. But, it only occurs following a copy and paste routine.
Thanks, Marilyn