Hi Ragdoll, and Welcome to us! ;D
This is most likely to occur when the PowerPoint file has been created in a later version of MS Office than the one used to read it.
First go to the MS Office update site
and click on Office Updates (4th link down in the left hand pane). If you still have the same problem after updating there may be 2 more updates you need, but I need to know which version of Office you are running (2000, XP, 2003 and so on). If you go to Start, All Programs and hover your mouse over MS Office it will say something like 'MS Office PowerPoint 2003'.
In the meantime you could download the PowerPoint 2007 Viewer
to view the files, though not edit them. Powerpoint produces files with one of two extensions - .ppt for editable files and .pps for 'read only' files. I'm guessing you won't need to amend your instructor's slides! ;)
(I had a similar problem recently although, perversely, the .ppt file had been created in Office 97 and I was using Office 2007 to read it).