I am having a problem with pretty low frame rates and I don't know WHY?

I am running MS FS2002 Professional on a brand new setup using Windows XP (home) on a Pentium 4 box at 2.4 Gig clock speed with 512 Meg of ram, a 40 gig C drive that is only about 10 gig full (also a 200 gig E drive not used for MSFS except for preliminary download storage), a Geforce 2 MX / MX400 64 meg video card, screen resolution at 1024 x 768, 32 bit, Direct X 9, and a Soundblaster Live audio card. All windows updates are current.... except for the one that was mentioned on this forum that slowed down the XP machine....which I deleted and got a tad better perfromance ;D.
If I head out into the wilds of the yukon with about one AI plane within 40 n. miles passing by at angels 12 and flying the default C172...... in an area of totally autogen scenery (no add ons in sight) ..... I am only getting about 15 FPS!!!!!!!!! If I go into Boston with a lot of AI traffic it drops to about 10 or so. I have it supposedly "locked" at 40.... but that seems to make no difference.
Just about all the simulation setup sliders are at full settings. I do not have anti aliasing on (it causes video problems if I do). Most other option check boxes are on. I have tried bi and tri level filtering.... no appreciable difference.
I see people here on the forum mentioning far lesser machines and getting frame rates that ar FAR better than mine with the sliders also all at maximum.
SO what the heck am I doing wrong???????
Any suggestions would be greatly appreciated.

best,
..................john