When I choose the Bombardier CRJ700 airplane from the free flight menu, FSX crashes.I can only stop the program with the Win XP taskmanager. This happens only with the Bombardier aircraft.Could there be a problem with the "aircraft.cfg"-file ?

No problems here. I would make sure I didn't change something with the CRJ files.At last resort I would do a repair install from the DVD.RhettAMD 3700+ (@2585 mhz), eVGA 7800GT 256 (Guru3D 93.71), ASUS A8N-E, PC Power 510 SLI, 2gb Corsair XMS 3-3-3-8 (1T), WD 150 gig 10000rpm Raptor, WD 250gig 7200rpm SATA2, Seagate 120gb 5400 rpm external HD, CoolerMaster Praetorian