help with my Laggy PC

My Pc has been laggy for about a month now and i have no effing idea what happened here are my specs
Radeon X700 (256 mb edition)
1gb of ram
2.1 ghz AMD processor
160 GB hard drive
250GB hard drive
its not the best pc but its a upgraded HP a706n i have the latest drivers and everything. I did tweaked all i could(startup,some registrys-this made is less laggyer) But everthing ido going on firefox even offline thing i use a wireless router that recives its signal from upstairs i run 3 programs at a time iTunes,GW,and AIM/firefox. But the problem i use Norton anti virus and i like it i do not know whats laggy could some one please help. I do not want format.

hit these 3 keys at the same time ctrl+shift+esc This will bring up task manager. Go under the processes tab and look and see what programs are using your CPU time the most. ALso check and see what is using most of your memory. have you ran a spyware cleaner and defragged your HD? Emptied out your temp internet files <offline files included>? Running iTunes or any music program that accesses the Harddrive will bog down your game a little bit. I do the same

turn off itunes and aim/firefox. firefox with everything turned on is incredibly laggy.

install a firewall of somesort. it's possible that you might have a few spywares/worms on your computer that's slowing it down.

lastly, what do you mean by laggy? do you mean the internet connection is slow, or do you mean that your framerate is low?

ive had a problem with lowered frame rates (not lag) for about a month. The thing is i've reformatted my HD because it was clogged anyways in that time and it still hasn't gone away. It seems like ALOT of other people are having this problem, so its prolly just something they did over the winter update. My theory is it has something to do with the viewer mode.