Excel acting up

It’s like the shift button would be pressed al the time, or as if I keep the left mouse button pressed although I don’t.
Once I select a cell, that’s it, wherever I move the mouse pointer the whole area from the original cell gets selected.
No key combination gets me out, cannot even access the menu.
Repaired AND than reinstalled office just in case, the problem has to be somewhere else. ALL other applications/programs/tools …everything else works without problem..