anything ending in.pps is normally a slideshow that can be opened by powerpoint.
Do you have microsoft office installed on your pc?
You say that you can open word processing based documents. Do you microsoft word installed or do you use notepad or wordpad when you open those files?
Sounds like you cannot open most files as you need to install MS office on your PC.
It's around $200 for the student and teacher edition, which is fine if you are using it for home use.
OR you can download openoffice for free (it around 100mb in size) or buy a cd with it on from DSE for $10.
Also if you do not have Adobe acrobat reader you should get that. It will let you open pdf's, again you can download it for free, or it comes on the openoffice CD which you can buy.
Hope that helps :)