Powerpoint Viewer won't open presentations

[possumbaby]

When I try to open any of my presentations on PowerPoint Viewer it seems at first to be working, with the "opening presentation" message appearing, but after about a minute the message disappears and it returns me to the "file open" box, but without ever actually opening my presentation. There is no error message, no indication as to anything being wrong, it's almost as if it forgets it's meant to be opening something!

This is a recent development - it was working fine as of a couple of months ago, and nothing on my computer has changed in the meantime.

I have Windows Vista and am using PowerPoint 2007 and PowerPoint Viewer 2007.

Any help would be much appreciated!