I have exactly the same problem with a Dell Precision M6400 under Vista ultimate.
All was ok until I reinstall my computer (because changing hard drive).
I've seen somewhere that video drivers could be responsible and I've updated them to the last ones from Nvidia website (for a quadro FX 2700) just today.
The problem still occurs, even if I have nothing launched (no ie nor ffox), just antivirus and some resident tools that I've tried also to stop without success.
And, in general, after closing all (if something is running), the problem disappear misteriously after a random time.
The pb just occurs now ... (I am on another computer) even though I'm not using the computer !
I've tried to kill WUDFhost but it still keep all cpu (taskmgr of course).
And I don't know how to tell Taskmgr to use only one processor.
If someone could help me ... I'm going to bo crazy !
PS : Sorry for my english ... I'm french !