I am getting the same problem as http://forums.arcgis.com/threads/19468-Opening-tools-causes-ArcMap-to-freeze-crash
That is, when I open any geoprocessing tool in ArcCatalog (might happen in ArcMap as well, though it hasn't to me yet), sometimes the application hangs. The timing of the hang occurs after the tool dialog opens but before it finishes drawing. I don't know exactly what conditions cause this to happen, but it feels random, maybe in 1 in 4 attempts it hangs.
For me, terminating the hung process and trying again sometimes works, sometimes it doesn't. But it's a huge headache.
Has anyone come across the same problem and found any resolution? I have already tried reinstalling ArcGIS. I think this started happening some time after installing SP3 for ArcGIS 10.