My guess is that you need to look at your startup programs, the number of memory resident processes actually running after startup, and also get a good program to maintain your regsitry file. Registry Mechanic or RegCure are both good.
Maintaining a healthy registry file and cleaning out unessential processes is as important as virus, spyware, and hijacking protection, and almost as important as having enough memory and the amount of processing you have. The things I have talked about here drag down what you have and so you need to make sure they are looked at.
Just as an FYI: This add/remove software not only removes programs, but also uninstalls any associated registry keys with each app. that is uninstalled. Maybe you've already heard about this, but I hadn't until maybe a month ago. Works pretty good:
http://www.download.com/Your-Uninstaller-2006/3000-2096_4-10619595.html
It will COMPLETELY uninstall just about anything.