Fonts are there, but Publisher says it's not and replaces it

I don't know if you'll be able to help me on this forum. I know your specialty is more the inner workings of computers, but I'll give it a try anyway.

At work we publish various resources for kids. Most of it is done in design software (like Adobe or Corel) but we are a bit backward in the teachers manuals as we use Publisher. All of our PCs are the same and have the same softwares but my boss's PC is being problematic with fonts when opening Publisher files.

All the font for the main body of type is Arial. Sometimes when she opens the files, it's fine but most of the time, Publisher (without warning) replaces Arial with either Times Roman or Avant Garde font. This throws off the whole layout, making it impossible to work on the file properly.

I'm pretty alright with understanding computers, maintenance and troubleshooting but at the moment, this issue is beating me. I wonder if any of you might have any ideas or suggestions.

Just to be clear, when publisher changes the font, she doesn't get the usual warning box. Also, even though the font has clearly changed, the font box still says it's Arial, when it clearly isn't Arial. I've uninstalled the font countless times and reinstalled it. Sometimes this solves the problem for a short time, other times it doesn't. Even when the problem seems to have been solved, a few open files later and it goes again.

It has happened a few times in CorelDraw, but not as painfully often as in Publisher.

The computers are XP. Actually, they came with Vista, I think but as we had the option of rolling back to XP we went with that. We do have an outside company who maintain our computers, but I don't think it covers this kind of problem as I think they only deal with hardware. I'm pretty sure this is a software problem somewhere along the line.

I'm sorry this post is so long winded, but I like to be thorough in my explanation. Hope someone can help.