Dell inspiron heats up shuts down

Hello,
My name is steve. I got a used dell inspiron 1425 used a few days ago. Was told nothing was wrong with it. Well with in about half hour of using it it heat up and shuts down. So eventually i figured out where the cooling system was. and posted a add on craiglist about help and i got the link to a forum about the same issue other people are having. So i blew the radiator and fan out with a shop vac but still gets on and the fan dont seem to kick on, and not sure when its not supposed to. So not sure if the fan is shot or what the problem is. So in order to run it i have to run some fans that blow air up underneith it just to keep it cool I really want to get this issue fixed because i really love this laptop. So can some one please help me out thanks.