I need help fixing an old computer for myself...please any advice

Hi, im trying to fix an old computer that we've had for almost 3 years. The computer turns on and works Its just that its really really slow and sometimes freazes. I need it mainly for school work(essays, research, or projects),music, just the basic stuff. I also don't want to spend too much, but what would you recommend me to do. I would really want to change the hard drive(but i dont know which brand would go good with what i need), i want to start fresh without buying a brand new desktop.

I have a Gateway GT5405E Microsoft Windows vista home premium