Firstly, I think u should go to the management tools of ur computer, and check it out whether there are too many unnecessary services installed.. Just deactive, disable the auto-running or even uninstall which u dont need..
Secondly, make sure u have closed all the applications running in ur system which u dont need when u in game, by this, i mean porn downloading program,
.
Thirdly, make sure u got the newest driver of ur video card and directx9.0.
fourthly, turn off the vertcal sync...
If all of these dont help u well, so come to my final solution, use the following in ur config file. But to ur attention, this would low something like effect of ur game, but defenitely impove the fps a lot...
mat_specular "0"
mat_bumpmap "0"
mat_clipz "1"
mp_decals "0"
cl_phys_props_enable "0"
cl_ragdoll_physics_enable "0"
cl_phys_props_max "0"
cl_show_bloodspray "0"
fog_enable "0"
fog_enableskybox "0"
fog_enable_water_fog "0"
r_propsmaxdist "1"
r_DrawRain "0"
r_dynamic "0"
r_shadows"0"
r_3dnow "1" // if u r using amd cpu, put this in.
r_mmx "1" // if u r using intel cpu, take this one.
r_3dsky "0"
r_WaterDrawReflection "0"
r_WaterDrawRefraction "0"
r_fastzreject "1"
mod_forcedata "1"
rope_shake "0"
I got amd64 2800+, 1g memory and 6600gt oc.. after all these 5 steps, I got like at least 70-80 fps in D2, and when no ppl around, its over 120fps..
p.s. If u are thinking about upgrading ur pc, my suggestion would be Get a better CPU... ur p4 2,4g is little bit weak in ur whole system.