Why do my desktop icons keep changing

When I boot to my Windows 2000 desktop, some of my icons (about 3 or 4) randomly change to some strange icon that I have never seen before.

If I do a properties and change icon, it shows the correct one, but it will not display the right one.

It affects any icon on my desktop.
For example sometimes my word icon is a white and black square, or my folder icon is a blue background and a strange piece of paper.

I am running an upto date virus checker, and it hasn't found anything.

Whatcould be causing this? I haven't seen this on Windows 2000 before and I have used it since its beta stage.