Have doubt of topic : screen or cpu

hi guys !

i am being irritated by a problem for which even choosing a topic is a li'l bit of trouble for me.

okay . now lets get straight at the point becoz i have no patience to wait to know the solution,

i am basically a gamer but i know my computer's limitations . (even some games released in 2005 lag much).

while i'm playing games like nfs carbon i get an average fps of about 12- 18.

games like gta san andreas give 15 - 20 fps but i don't mind them becoz my rig is :

intel pentium t2450 2.00 + 2.00 ghz
2 (1 x 2)gb ddr2 ram
mobile intel express chip set family (256 mb , dedicated - 0 mb) graphics.

things got worse 1 year ago , when i'm playing games i can't run any other program (like utorrent or opera15 or even windows explorer) otherwise the screen will become dark and after a few moments it goes to hibernate or shuts down improperly.

but things got even worse recently , when running both opera15 and utorrent led to the above mentioned consequences.when i got my hands to the side of the computer it was damn hot.

i don't know whether it's a cpu overheating problem or screen fault.

should i buy a heatsink or cooling fan or vacuum the dust inside my little beast ?

which is safe and efficient ?

and what should i do to increase the smoothness and fps of games ?

those who have a good solution are most welcome to reply.