Computer uses all my RAM even when I'm not running programs

My computer is no longer able to run the most basic programs on my computer without eating away at all my RAM. What really gets me is how slow my computer starts up as well. I can assure you I've gone through my start up folder and removed the programs that were on there, as well as changing settings of certain programs so they don't start up when I turn on my computer.
I'm wondering if there might be a virus or something causing all this.
I've read a previous thread on this topic, but I need help in making sense of my hijackthis report and what I should do to remedy this problem.