Windows 7 64bit.
I am using ProcessExplorer from Sysinternals, and it says, that the offending call is
however, the call stack towards the entry is always different, so it's hard for me to track the problem. Maybe it's a mal-programed trojan, causing exceptions in Explorer.exe, but that is only a wild speculation.
Explorer.exe is then consuming 25% (a core on a dual core). Killing the process makes the task bar go away, respawning from task manager, and half a minute later it's again eating all CPU cycles.