am assuming that you are using windows xp
what anti virus are you using please?
is it only gaming that is affected?
looking above at the post i made on anti spyware, have a look and see if that helps please
also, lets have a look at what is loading when you start the PC - some of those things can slow stuff down a lot so, click on start button, then run, and type msconfig and press enter/return
click on the start up tab and look at what is loading there - there will be shitloads btw, this is normal. The only startup items you absolutely need are:
ScanRegistry
TaskMonitor
SystemTray
LoadPowerProfile
Your antivirus program (trust me, you won't have a hard time figuring out which one is your antivirus program. It is pretty obvious!)
next up, lets measure the actual speed you are getting - head to
http://www.thinkbroadband.com/speedtest.html and see if that measures up
next if there is nothing wrong there, and the speed is correct, and you have no obvious bandwidth hogging software that is running all the time, lets look at the actual ping rate -
http://pcpitstop.com/internet/default.asp has a nice little tool for that, be aware though that your firewall may well be blocking it! For this test only, and do not have anything else running, disable your firewall and run the test.
Let us know how you get on with all those - I suspect that it is your firewall that is slowing things down, or the modem/router you are using is not quite right