I have received a few Xmas cards using powerpoint. Is their an Ubuntu equivalent to open this file?
Ubuntu comes with Libreoffice, an office suite that replaces Microsoft Windows. The Libreoffice equivalent to Microsoft PowerPoint is called Libreoffice Impress. In any case, you should be able to open you PPT or PPTX files by simply double-clicking the file.
By the way, in order to get to know the alternative to other popular software, I recommend takign a look at http://alternativeto.net/. Here's a list of alternatives to PowerPoint in linux: http://alternativeto.net/software/microsoft-powerpoint/?platform=linux