Revit 2011 (Mental Ray) utilises all available CPU cores that the OS can 'see.' When multitasking, your PC may slow to a crawl after a render begins. You can limit the amount of CPU time that the Render process sees by either:
Adjusting the 'Affinity' to limit the amount of cores the render process can access (using Task Manager).
Adjusting the 'Priority' to Low to allow other processes to have more overall CPU time.
Simply open Task Manager (Ctrl+Shift+Esc) and then right-click on the render process (fbxooprender2.exe). You can now adjust the Affinity and Priority.