PowerPoint doesn’t treat hyperlink underlines as regular text underlines. Learn how to remove underline from link in PowerPoint using this guide.

It's an autocorrect option, you can disable the behavior there. Also, at least in 2000, when you right click on the cell one of the options is "remove hyperlink".

How to control a hyperlink’s appearance in Microsoft Word Your email has been sent Hyperlinks are a part of our everyday lives—we readily recognize the underlined blue font as a link to related ...