If you're getting messages like "PowerPoint cannot open the type of file represented by .ppt" when attempting to open a PPT file, it's likely that:
Tips: How to Recover Corrupt PowerPoint Files
Your file was password protected in PowerPoint 2002 (XP) but you're opening it in an earlier version of PowerPoint; earlier versions can't open password protected files and will give this scary error message when you try. Mac versions of PowerPoint through 2004 are also unable to open these presentations.
Your presentation file has become corrupted
If it's a PPT XP file and you're opening it in an earlier version of PowerPoint, have the file's owner open it again in XP and save again without the password protection.
Otherwise, getting a corrupt presentation back seems to be pretty much a crapshoot. If you have an earlier version or a backup of the file, dig it up now. If not, you can try some of the things listed below.
Whatever else you do, make a backup copy of your presentation and work on it. Never try any of these recovery techniques on your only copy of a presentation.
Things to try:
If you run PowerPoint 2003, choose Help, About Microsoft PowerPoint from the main menu bar. If the About screen doesn't indicate that you have SP1 (Service Pack 1) or higher, you should update your copy of PowerPoint. Choose Help, Check For Updates. When the Office Downloads page opens in your browser, click "Check for Updates" and apply the recommended hotfixes and/or service packs.
If you run Windows XP and have installed SP1, see Files open as Read-Only after installing Windows XP SP1, Other file/path related problems
Open a blank presentation (You can apply the template used for the damaged file, if applicable. That might help cut down on reformatting.)
Insert, Slides from File
Try to insert slides individually if you're not able to insert the whole thing
Sometimes you can get some of your work back that way.
If your file corrupted during, say, a computer crash, you can sometimes locate a temporary version of the file in your TEMP directory. If so, you can try the aforementioned Insert, Slides From File and browse to that file. Or you can try renaming the extension to PPT and see if you can open it from within PowerPoint using File, Open. You could also do a search for *.TMP files on the off chance that it's not in your TEMP directory.
Troubleshooting damaged presentations refers to Windows 95 and PowerPoint 97 specifically but may have some useful suggestions.
If the presentation seems corrupted or on the verge of it, but you can still open it, download and use the CloneMe add-in from Microsoft (for PowerPoint 97 or 2000).
Many users have recovered damaged PPT presentations by opening them in Sun's Star Office presentation package.
Or try Open Office, the free version of this same package. Note that it will ask if you want it to take over registration for the various Office file types (meaning that when you doubleclick a PPT file from then on, Open Office will launch instead of PowerPoint.) Unless this is what you want, watch out for it and just say "NO"
There's also Office Recovery, a set of expensive commercial tools.
If nothing else works, try opening the presentation in Word to see if you can at least recover the text:
Start Word and choose File, Open
Choose file type "Recover text from any document"
Edit the file as needed