What are the main reason of slow PC??

Hi
i have my own PC and i experienced that the PC become slow in some days after setup of operating system.
I mainly used Windows XP professional service pack 2 its seems to me that it become slow too quick but in Windows Seven its not that much quick.
Now my question is that what is the reasons behind this problem. There should be some good reason of this problem and what are the remedy of this problem.
I need to know because its really frustrating as i have a good configuration in my PC. Though i have a good configuration my PC gets slow so i need to know the reasons behind this problem and how to stop this to happen again.
some one please tell me how can i overcome this problem…
i will be very happy if you please explain the way should i go to overcome this problem.
thanks
