From your first post, the computers were imaged with FOG back in July. If they recently started to experience a slow performance then it will not be FOG. On the systems in my office FOG does not take up more than 7MB of memory. I could be wrong but IE8 should work fine on a single core. I would recommend if you use IE to upgrade them to IE9 since it is more secure. In my office I give each user the choice to use IE, Firefox, or Chrome (my preference). All are included in my images.
I would check the Task Manager to see what process is slowing down the machines. I am guessing they are using 1 GB of RAM which should be okay. Depending on how old they are, follow rixter’s post. Especially open them up to see how much dust is in them.