Why is my macbook running so slow?

I noticed that my mac book was running slower than normal. I had recently downloaded alot of HD video off my video camera so I had thought that was the reason. I installed a new 250 gb hard drive and upgraded to 2 gb of ram and it did not improve at all. It takes a while to load a page using safari, sometimes it freezes and I need to restart. My imac that is connecting to the same network runs super fast so it's not the connection.
Any Idea's?