For what it's worth...
My 6y/o runsXion on my old machine
an Athlon XP 1700+ (which runs at~1.2GHz) /XP+SP2 with 512MB and Realtec onboard Soundcard to listen to the Wiggles
while playing games. So your computer should be more than able to do the same.
Seems to me that it's just a settings problem.
Does that only happen in connection with FF?? (if so, you might want to google the appr. "tuning"-settings for FireFox (via about:config)
If that's a general problem, I would say, your machine needs some fine tuning
I recommend identify the source of the high CPU load first via taskmanager or process-manager (much better b/c it shows more info) and go from there.
You could always make sure to get rid of as many unnecessary background tasks as possible (via msconfig)
and reduce the HDD asctivity by setting the pagefile to
MIN and MAX to ~1.5x your RAM (1536MB)
Google is your friend
let me know if you need closer instructions
-DG