I installed the game and played through chapters 1-6 just fine...However I have come across the first gel section..well in the 2nd room of the blue gel,you know that cube you have to grab before bounceing back on the blue gel to put it on one of the Old apaeture buttons....whenever i drop the cube...portal crashes...

Oh and Multiplayer crashes now as well. Help?

On Steam's games tab, right click Portal 2 and then select Properties. On the properties window, go to the "Local Files" tab and press the "Verify Integrity of Game Cache". It will replace any broken files your game might have. Can't figure out anything else now, so tell me if it works.

Post your system specs.

Thanks Guys, the steam verfiy worked like a charm along with a system update.

Not sure what my specs are.

i have the same problem where could i find Steam's games tab.please help me

Try the suggestion mentioned above.
And if that doesnt work post your specs.