It could be a sub-process or application that's running in the background
and taking all the CPU resources.
To find and display what could be the problem try Process Explorer:
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
Once you have Process Explorer installed and running:
In the taskbar select View and check 'Show Process Tree' and the
'Show Lower Pane' options.
Then expand the process named 'Explorer' (click on the + sign)
In the column on the left named 'CPU', look for any high CPU usage.
Next click on the CPU column to sort the processes by %CPU usage
(Highest to Lowest).
Move the mouse cursor over any process,
you should see a popup with some detailed info.
Then mouse over the process that's using most or all the CPU %.
Then click on that process to highlight it,
Now that it's highlighted, right click and from the options listed select:
'Search Online'.
This should display what out there on the web about that process.
You can also double click on any process to open up a more detailed
'Properties' window.
Note: some entries like Explorer, System/Services, and Svchost
may need to be expanded to show the detail (sub processes),
in this case click on the + located to the left of the entry.
An alternate method when using Process Explorer
is to double click on the Graph just below the Menu bar.
This will open the 'System Information' window, which has a larger display
of all three graphs. Move your mouse over any spike in the
CPU Usage graph to see what process/application or service is the cause
of the spike.
