Ok, this one is weird. I have a user with Office XP SP2, full install of Excel.

When he types in 500000 for instance, no comma appears.

Great, I says to him, I says "That's just a formatting issue."

So, I go to Formatting Cells and go to the Number category.

Hmm..."Use the 1000 seperator ( )" option is already clicked. Check this out
though, there is no comma in the text description, just like I typed it
above. No comma.

It's like the comma image ain't there, it's been totally blown away.

Any suggestions?