It shouldn't do that. It will heat up, but that shouldn't slow it down unless there is something else going on.
I'm guessing that you have a program that is constantly running, like an Antivirus program or a real virus.
Doesn't it make sense that a program that is using the computer constantly would 1) slow it down because it is using resources and 2) create a lot of heat because it is using the hard drive and the processor constantly.
In Windows XP, close all open programs and windows.
Press Ctrl-Alt-Del and select Task Manager if it doesn't pop up automatically. Click on the Performance tab and look at the CPU usage history. It should stay down pretty low. Also look at the Physical Memory. Is this less than 50000?
Next, click on the Process tab at the top and then click on CPU. Watch the top of the column to see which program is using the CPU the most. The column is giving a % of the CPU used by that program. Do the same for the "CPU Time" column. Write down the top 5 programs that are using the CPU. Write them down EXACTLY as they appear under the Image Name column.
Then go to http://www.processlibrary.com/ and type each one in to see what program it is.
For example I type in "rtvscan.exe" and find that:
"rtvscan.exe is an executable of the Symantec Internet Security Suite. It is responsible for the execution of real-time virus-scanning in order to detect virus infected files as they enter your system. This program is important for the stable and secure running of your computer and should not be terminated"
I think you will find that something is using a LOT of resources and heating up your computer. Ironically it often is Norton AV or McAfee. You can turn of some of the monitoring features to keep them at bay.
I hope you quit getting burned.