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I'm on XP . . .
So if this is not appropriate I apologize. And who's to say what's possible with Vista

I've had things like this occasionally.
Check your file associations - 'My Comp/Control Panel/Folder Options/File Types'
See what it shows there - if that looks good - reboot and see what that looks like.
If it still goes out of whack, it could be a sign of some instability in the system.
You could try some maintenance things like defrag and clean up the files.
I use Defraggle and CCleaner - don't forget to make a 'Restore Point' (before running CCleaner)
If that doesn't do anything, it could be corrupt files in the operating system.
You might need to run recovery. I wouldn't rush into that. Could be dicey.

I got a lot of that with the old 98, A bit of it with XP but it's never permanent.
And - I feel my system is quite stable at the moment.
I usually happens because I have done something out of the ordinary. And it is usually fixable by resetting the file types.

Hope this helps - dz

Which Operating System are you using?