There is a user in my office who is having a strange problem in Windows, and I'm not even sure how to search for it.

When he's in a new email in Outlook, he'll go to attach some files. The attachment window opens, and he'll go to a folder with a bunch of PDF documents in it. He'll select the first one, then he'll go down a bunch, hold shift, and click on another one. The problem: for some reason only the last file clicked is being selected. The initial file becomes deselected, and none of the files in-between are selected. The same basic thing happens if he tries to Ctrl+Click documents. Only one document can be selected at a time.

Does anyone know what might be causing this?