Sometimes Rhino will misbehave or even refuse to start. If you believe that there may be a plug-in involved, or if Tech support has suggested that this might be the case, then selectively 'load protecting' plug-ins and trying Rhino with these plug-ins not loaded and unable to load without your permission is a good way to zero in on the culprit.
To load-protect plug-ins, open Options > Plug-ins page. There you will see a list of the loaded and available plug-ins. (you can customize the list to excude, for example, Import/Export plug-ins which are the majority and unlikely to be causing problems, to make it easier to locate the ones you are looking for- look at the drop down list in the upper right)
Each plug-in has a check box for load protecting- if you locate the plug-ins you want to test, check the box next to each, answer the confirmation question, then restart Rhino. When Rhino starts, it will ask when it wants to load the plug-ins rather then just loading them normally- if you say 'no' to to this question, the the plug-in will not load and you can test whatever it is you were doing that was causing a crash and see if it still crashes. By protecting and unprotecting plug-ins you can zero in on the one that is causing trouble.
If Rhino refuses to start at all, then try starting in 'Safe mode' - there is a Rhino in Safe mode icon in the Windows Start menu > 'Programs' entry for Rhinoceros 4.0 . Starting in Safe Mode blocks plug-ins from loading; you can then load protect the ones you suspect and then restart Rhino normally.
Typlically the first plugins to try to load protect are third-party plug-ins like renderers, CAM products, beta software like Brazil, and in some cases the Rhino RDK plug-in. If you find that Rhino behaves better without all of these, then try enabling half of them at a time to try to locate the one that causes trouble.