Windows 7 64bit.
I am using ProcessExplorer from Sysinternals, and it says, that the offending call is
ntdll.dll!RtlValidateHeap+0x170
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.
Best Answer
Use SysInternal's sister program, Process Monitor, to see what it's actually doing.