My computer keeps shutting down by itself.?

I have a windows vista xp . I turn on the computer..I wait and let it start i log onto an acc and after trying to find ways to fix it it turns off in like 10min. I have to wait like 5more min because if i try to turn it on again it shuts off right away. So when I do restart it, it says that the computer turned off because of overheating. So I bought a spray called Ultra Duster industrial strength(compressed air in a can) to clean the fans cause people said there might be a lot of dust on the fans, which there was and took of all the dust. I tried to turn it on and i thought it worked it took a little longer to turn off but it still did and kept repeating what i said above. So then I kept looking for more ways to fix the problem an people were saying it might be your RAM i checked and said i only have 512mb ram is that good or should i get more memory, if i do get more memory how do i install it, how much will it cost. Or if this happened to you please tell me what you did thanksss.