Hi,
I had same problem ... i tried everything .... cleaning registry, deleting everything related to ESRI and ArcGIS, renaming folders in registry and app data, system restore ... even clean install of ArcGIS ... nothing helped ...
Then, i have researched the problem and I have realized that problem was in upgrading of Internet Explorer to v11 by default windows updater
So, I downgraded my IE to v 8.0 and I have set up security for ActiveX controls and plugins following instruction on this thread ArcGIS Geoprocessing Tools Scripting Error GISNuts.com
And whoa toolbox started to work like charm again...
I hope that this will help someone