.net framework 3.5 setup does not start

i can't start any .net app, every time i get the normal error dialog that the program has found a problem and needs to be closed. i tried to uninstall all .net versions, and then reinstall the 3.5 .net setup, but when i double click the exe nothing happens. just the UAC dialog appears where i can choose yes, but then nothing happens. i redownloaded the file 5 times, but it does not work : ( and there is'nt any process in the taskmanager also

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