you dont say if this is a spefific jpeg or allof them, try right clicking the file and go to properties , on the general tab check what program its set to Open with... if that isnt the answer its beyond me probably a little gremlin in the registry.
I get a similar thing with Word2002, quite frequently it tells me it wants to install a new component when opening a file, that I've opened with no problem before. I have to insert the set up CD to keep it happy then everything is OK for a while. Maybe something similar will help in your case
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