The most common problems of low framerate could be:
-Low Ram memory or too much apps open.
-CPU are being used by another app or is not enough powerful
-Graphics card is not enough powerful
-You changed the rendering system
-Your computer is being evil.
To check the available Ram memory press ctrl+alt+del and open the
task manager, check that the
FP Usage is not too high.
This pic shows a medium Ram and CPU usage but enough to play FH without problems.
If it is too high try closing every application.
In the same task manager check for the CPU usage to be low when you are not running FH. and again close any application that uses it.
As your computer is brought in 2004, is probably is too old for FH, my tip is go to maps with not too much people.