When running one of our components in the visual studio debugger, the symbol server hangs downloading rasapi32.pdb. The problem seems to occur when our component is loading the WebBrowser control. We started seeing this hang after upgrading to Windows XP SP3, though I think that's a red herring and the SP simply upgraded some DLLs that now need new symbols. There also needs to be some content in the control (about:blank doesn't trigger the problem) to get a hang, but again I think that's just because one has to get IE to load some DLLs I don't already have symbols cached for.
I believe this is the same problem that Steven Wilssen replied to at http://social.msdn.microsoft.com/Forums/en-US/refsourceserver/thread/26a4b712-6c82-4627-90f8-7188c7ebab10/, where he indicated he was seeking a repro case for this. However, my attempt to contact him bounced with "Recipient address rejected: Access denied"
My theory is that this is a deadlock in trying to fetch http resources:
1) the embedded is loading a number of helper dlls as it downloads the page content;
2) the symbol server fails to find cached symbols for at least one of these (as in the linked forum post, the one that hangs for me is rasapi32.dll)
3) it then tries to connect to http://msdl.microsoft.com/download/symbols, but ends up waiting for something internal to wininet/urlmon that is already held by the web browser control's incomplete download, which is not making progress because the debugger has stopped the process to load symbols.
IE also hangs trying to browse to any site while the machine is in this state, but Firefox still works, so I think it's some sort of system-wide lock in wininet HTTP handling. Lacking much knowledge of wininet internals, that's the best I can do for a theory.