It can be a number of things but before you crack open the pc and start cleaning it (Watti not everyone knows whats what INSIDE a pc )just check if its not SOFTWARE thats the problem (I think its software !)
A few tips : 1 check the processes you are running (rihtclick on the taskbar select taskmanager look in the low left corner it says :Processes nr , CPU use nr , memory use nr
now look at the processes , how many are running , whats the CPU usage (its a percentage of your total processor power) and look at the memory use
now if you have a lot of processes running (roughly more then 50 /60 ) this coiuld use a lot of memory AND /or cpu . If you see that the cpu usage is high EVEN idle just at the desktop , you might just have installed a lot of toolbars etc or are running a aplication inthe background . ! but it can also be a firewall that is using a function that is eating up all your cpu . It realy can be a lot of things .
Are you on TS , because that is a great way to give support , now I am no Expert , but i have decent hard /software knowlage and I can probably help . In the case I can not I think Sickness is more than happy to help you on Msn , but trust me ts goes super fast (if your pc can still run it ?)
One tip before hand : are you one of those guys who never shuts the pc Down ? this is not good , a lot of processes are automaticly started when you run programms and are not always shut down after closing the programm , It is therefore important you REBOOT your pc regularly . this shuts those processes down
Learning the use of the taskmanager is very important , in most cases it helps you find the problem .as it tells you what processes are using the cpu / memory and if they are in fact using it all .