Hi guys,
From what I've seen, most of the time when running "ordinary" programs such as your Office package, web browser etc. your processor will indeed be "idle" for 98-99% of the time, with the odd "spurt" when it has to do something. I run 2 monitors so I can have the Windows Task Manager open on the second monitor and see what's happening in real time.
However if FS2004 is running, when active, your processor usage will probably sit at 100% for most of the time !
Also be aware that the "dynamic memory usage" should remain the same when you open and close applications, but in practice it doesn't (in WinXP anyway.) This means that, for instance, freshly booted, my PC will show around 500-600Mb RAM but after a few days of use this will creep up, heading for 800-900Mb or more, which will obviously impact on the available memory for FS2004. With the likes of UT and real weather (Active Sky) I frequently see well over 1Gb RAM use before I even start flying, and if I load FS2004 with the PC already using nearly 1Gb RAM this can be up to 1.5Gb before flying starts. This leaves only 500Mb "free" memory to be "eaten into" before I hit the ~2Gb maximum Brick Wall and the inevitable "out of memory crash".
So, if you don't already do so, you may find freshly booting your PC before starting an intensive simming session gives you some extra 100s of MB of RAM to play with which may help to prevent this crisis point from being met.
Hope this helps,
Alastair