unable to open files on network drive

[sed...@officeformac.com]

Version: 2008
Operating System: Mac OS X 10.5 (Leopard)
Processor: Intel

I have files on a Mybook world (NAS drive) on my local network. When I try to open powerpoint files on the drive I get one of two messages:
for ppt files - Powerpoint cannot open the type of file represented by (filename)
for pptx files - Powerpoint cannot access the file (filename) because the file has been moved or deleted.
The odd thing is that if I copy the file onto my desktop I can open it without any problem.
I can also open the files with Keynote and Office 2007 (Windows) on the network drive.
Anyone know of a fix?