You say you are not very good with computers.
I am afraid this is a situation where somebody who DOES know about computers could probably sort it out. Do you have a friend or relative who can help you ?
There are some things I could suggest, but it would take too long to explain them all in this forum. Also, if you delete a file or program that you think looks bad it may stop Windows working.
If you are using Windows XP or Vista try the following.
Press the Ctrl key AND Alt key AND Delete key all together.
You should get a Task Manager window.
Select the Processes tab. This basically tells you what Windows programs are running.
Click on the CPU column. This tells you which programs are running and using up the CPU.
Normally if the computer is idle the System Idle Proces is top with about 99% if the computer is not doing anything.
Are there any other programs in the list that have a number in the CPU column ?
If so it may be the program that is making your PC go slow.
If there is a program dont do anything with it, just let us know what the program is.
Close the Task Manager window by clicking on the top right.