Slow response on computer...

Me and my wife is got a Dell Dimension E520 and I want to describe what is going on.

Sometimes the response is very slow and does not run as fast as it should. But here is the thing. We upgraded to 4GB RAM memory and got a 1 terabyte hard drive as a slave drive. Before the machine ever had this, it was running real good. But when we upgraded, the machine kind of runs slow. Is there a way to either upgrade the BIOS, or make some mild modifications to it so it will work a whole lot better?

Just curious if this can be solved...