HTML Menu Tool only works on some PCs

The CMS Builder was used to create the website that I am now maintaining by another company. It all works quite well except for one or two small problems.
When I am editing a page I click on the HTML icon and the HTML code appears as it should but for only about 3 seconds and then goes back to displaying the regular page. And when I hover the mouse over the <> Icon to View HTML Source, the icon shifts slightly to the right - see attached.
However on a friend's Vista laptop it works perfectly??? I tried this on a few other WinXP PCs and mostly it does not work.
I don't know what to reload or fix. Can anyone help to make a suggestion?
I would go back to the original developer but they are being difficult.
BTW - the icons at the top of this "Post New Message" page work perfectly on any PC I use.
Thank you!!