Yesterday i re-installed all CS4 software (as suggested by Adolfo Rozenfeld) and rebooted my MAC. AE and Illustrator still crash at start-up.
Today, i installed Siverfast HDR (a Photoshop plug-in). By mistake i started After Effects in my old account. Coudn't believe my own eyes : it was working ! Tried Illustrator : working too! Note that the photoshop plug-in is a free demo plug-in. As desperate people do desperate things, if you are experiencing AE crashes at startup, you might try this too... who knows?