Hmmm... there's almost identical rig at my parents', on which I had my first touch to FreeSpace 2. It has an AthlonXP 3000+, 512MB DDR-RAM and a Radeon 9200SE (or LE or whatever the crap they named it, isn't really important anyways
).
And whaddaya know, I vaguely remember something similar with the retail being slow in the beginning. Praytell, do you perchance have Aureal 3D sound selected as sound device from the retail launcher settings? Try playing around with that, I don't remember which option fixed it but the default was Aureal 3D, and it didn't cut it. I suppose DirectSound was in the options list, or something like that... I haven't played retail FS2 on that PC in ages.
Of course, you could just DL and install latest version of FS2_Open... you won't be able to use all of the MediaVP's, but it should at least run. And if you're willing to sacrifice some FPS you can get some of the eye candy working as well... at least on 1024x768 resolution. The 9200SE seems to have problems running the HTL graphics on higher resolution...