Quite a long time back I had the same problem but using Windows Mail. .PPT/.PPS extensions would open if I saved them first, but not if I clicked on them in an email.
I believe it was this article that solved the issue, but as I said it's a while back: http://www.winhelponline.com/articles/149/1/Unable-to-open-PPS-attachments-directly-from-Windows-Mai...
I don't think VirusScan is stripping anything, I think this is a Windows issue.
If VirusScan is doing it them there should be a record in the Security Center
Double-click the taskbar icon to open SC
Click Advanced Options (bottom left)
Click Reports & Logs (left)
Click View Log (right)
There should be a record in the Email & IM item on the left.