Will, the question is very broad and depends on a lot of variables. When my clients come to me with this complaint I start with two things.
1) I gather all of the specs for that client's system. You're already listed the PC here. But if you've modified it at all, that would be useful to know.
2) What activities do you principally use your machine for? What are you doing mostly? Are you a worker? What kind? A student? Do you research? Do you play games or media? Are you a designer or programmer? Are you working on creative output (photos, movies, web)?
Also, there's a third think worth knowing, but it involves some observation on your part. Is the poor performance general, all the time? Or is it generally when you are doing certain things?
Starting with good info and more folks can likely help you out. To be honest, with my hands on scores of computer a week, the most common cause of slowdowns I professional see has to do with browsing. The web is a resource-intensive place, often more so than most games even. Better managing your system, better preparing your browsers, and having better and more attentive online use behaviors will go a long way towards most performance problems.