Currently Being ModeratedApr 7, 2013 6:11 AM (in response to juergenblo)
Welcome to the Apple Support Communities
Try downloading Mac OS X 10.6.8 from the Support page > http://support.apple.com/kb/DL1399 If you receive an error, tell us what the error is.
It's possible that you are running a pre-release Snow Leopard version. Did you install Mac OS X 10.6 from an official Snow Leopard disc?