|Can you also try doing this but saving to a different location (do you have an external HD or USB thumb drive)?|
The next thing I'm going to suggest is that one of the graphics or fonts you've used in your presentation is corrupt and is preventing the file from being saved.
Try this first and see if it helps:
1) Go under the Format Menu
2) Choose Replace Fonts
3) No matter which fonts you've used, replace them all with a standard one (Times New Roman) is probably the safest.
4) Try now "Save As" under a new name
You may see how many solution about here:- http://www.filerepairforum.com/forum/microsoft/microsoft-aa/powerpoint/303-my-powerpoint-presentation-didnt-save