Single threaded applications such as Prime95 will only cause a maximum CPU usage of 50%.
We performed two different tests in order to illuminate multi-tasking performance. The first one runs Doom 3, while Lame 3.97.1 multi-threaded encodes a large audio file into the MP3 format in the background. In a second run, we add a WinRAR 3.4 instance compressing a 1.2 GB test file, in order to further increase the workload.
Since Doom 3 is a single-threaded 3D game, it is not too difficult for the Windows scheduler to maintain high frame rates as long as the focus is on Doom 3. In order to create an even higher workload, for the second test we switched Doom 3 for an application that is already multi-threaded and is able to exploit dual core performance. We picked 3DS Max 7 here, and repeated the tests with either Lame 3.97 or Lame and WinRAR 3.4.
We also played around with different services such as anti-virus programs. At the end of the day, however, we found that they are only useful to show the inability of single core systems to maintain responsiveness. Any dual core system will easily be able to cope with additional loads to a lesser or even more significant extent.