When i play toribash, my frames are very instable, i can go from 19 to 61 but it keep changing, i would like to know if there is anything i can do to always be at 60 fps

Sometimes even when I close everything the cpu usage still shows like 80%

(toribash,music and facebook open)
pls

i have a old computer dual core 2 duo 2.66ghz
8go RAM
winXP

i think it might br windows xp playing a role here

i know but he is so KAWAII