Macbook Pro running slow, turns hot, fan blazing, and Finder not responding after trying to open a folder

I have a Macbook Pro 13" with Lion OS recently I updated my mac software to 10.7.3 and this problem has occured to me since yesterday. I have tried to move the folder to my external hard disk but when I try to open that folder, this problem occurs again. It only happens to one folder only, my school folder and the rest no problem in opening it. Anyone please help! Thank you so much.