Minecraft is Really laggy

Minecraft is Really laggy

Added (1). I'v recently bought a computer of a mate and i can't play minecraft without lagging(5/10 fps)! I have installed optifine and optimized the settings but to no avail. How can i stop minecraft lagging?
Processor: Intel(R) Pentium(R) D CPU 3.00GHz 3.00GHz
RAM:3.50GB (3.22gb usable)
System Type: 64-bit
Graphics Card: ATI RADEON XPRESS 200 Series
Any help would be greatly appreciated, Thanks.

Went from 200+ fps to permanently below 50 after laptop got very hot

So the other day I fell asleep after using my laptop but forgot to close the game I was running (minecraft) before I fell asleep. I was on my bed using it so anyone who has a gaming laptop will know that they get no air filtering through when on a bed/blanket, so when I woke up the thing was really hot. After waking up I turned if off and didn't use it till the next day. No every time I play I can barely get 50 fps, which I know may be perfectly fine for some people, but going from 200 to 50 permanently means something has gone wrong and I want to try to reverse it, so please if anyone could help that'd be great!