Did you reboot Windows?
If that doesn't work, re-associate your .jpg with the picture and fax
viewer and see if that works, then re-associate with CS4. Yeah, I'm
grasping at straws, something windows users are used to. ;-)
If those things don't work, try the image editor:
http://windowsxp.mvps.org/imgeditor.htm
I've got my .jpg set to open with picture and fax viewer but the above
program adds Edit, which opens the .jpg in CS3.
--
JD..