Codecs, like DivX, XviD always come in useful when you need to view stuff.
Also, for that matter, you could just install FFD Show, which renders videos and acts like a sort of universal codec for many MPEG-4 files, an ever more popular format for various video files and soon becoming a must have for every computer.
Even though its a PC that's mainly office based, its always a good idea to have your computer able to open a wide range of popular files...you never know what events may crop up when you want to view a file quick (e-mail from friends being the highest reason for this).
FFD Show