when the Grasshopper installer runs, it deletes all previous components installed for 0.7 (in 0.8 there's a new folder "\libraries" that is kept intact).
So, as Andrew says, it should be enough to uninstall Wb and then reinstall it. Does it help?
yes..that's exactly what I did after the reply of Andrew. After uninstalling and installing it again, WB runs perfectly :) Thanks to both of you for reply! I haven't really used WB, just checked out the examples in the instalation folder, but still wanted it to run on GH 0.8 just to play around with it some time ..