What version of PowerPoint are you using? X or 2001? Regardless, have you
applied the latest updates?
Office X:
* Microsoft Office v. X 10.1.2 Update
Office 2001:
* Microsoft Office 2001 Service Release 1
* Microsoft Word 2001 for Mac Security Update: Macro Vulnerability
* Microsoft PowerPoint/Excel 2001 Macro Security Update
* Entourage 2001 - Hotmail Updater
* Microsoft Office 2001 SSL Security Updater
You can get all of these from:
<
http://www.microsoft.com/mac/downloads.aspx>
I'd also recommend updating to the latest OS, which you can get from
Software Update.
It might also help to repair permissions, if you are using PowerPoint X.
For OS 10.2.x:
Go to Macintosh HD/Applications/Utilities. Open up Disk Utility. Select
your hard disk, then click the First Aid tab. Click the button to "Repair
Disk Permissions".
For OS 10.1.5:
Download the Repair Privileges utility, free from Apple, and run it:
<
http://docs.info.apple.com/article.html?artnum=106900>
If that doesn't help, try moving ~/Library/Preferences/Microsoft to the
Desktop, then re-launch PowerPoint and test. If it works, you can trash the
folder. If not, you can move it back. (in PowerPoint 2001, this is located
in HD:System Folder

references:Microsoft)
In the future, please state what version of PowerPoint you're using.
(~ = Go -> Home or HD/Users/<Your User>/)