Got Trados v.7.5 instaled on a Vista OS, been having trouble opening ppt files even those from Office 2007.
Well, you gave the answer already!
Trados 7.5 doesn't support Office 2007.
And I'd be very sceptical to run it under Vista (even though Microsoft says otherwise).
7.5 was not developed to work under Vista, so any kinds of bugs could appear.
Try sending this in to the SDL support. They'll probably tell you that your setup doesn't meet the system requirements...
Better downgrade to XP again