Windows 7: Choosing Default Program opens the wrong program

In January, I set all JPEGs and TIFFs to open with Photoshop CS5. Everything has been working fine until recently. I double-clicked on one of my JPEGs, and it opened in Windows Photo Viewer.



I figured I must have somehow changed the defaults, so for the next picture I used the "Open with..." method. I browsed for the Photoshop CS5 EXE and selected it, but in the program list, Photoshop CS4 showed up. If I actually select it, Photoshop CS4 indeed opens.

What makes this stranger still is that when I navigate to the same location on my C drive, Photoshop CS5 is there. I can open and run it with no problems.

I see other people posting that they can't select a program or that their choice isn't remembered, but I haven't seen anyone saying the wrong program is opening. How can I fix this?