minecraft lag

i have a nice pc but i cant play minecraft without lag can someone tell me why?

Intelcore i5-450m
Nvidia GeForce GT 540m
4.00GB Memory

I think you have windows 7 right?

You could try to close useless and not needed programs.

yes, i sis that already still the same

You could try Gamebooster: http://game-booster.nl.softonic.com/

Don't know if this will work, You can also try to set the setting low on minecraft.