Did you try to copy a set of custom shorcuts from an earlier version? Taht might work from CS5 or 5.5, but earlier might cause this kind of trouble.
If not, perhaps your prefs are damaged. See Replace Your Preferences
I have replaced my preferences twice, two different ways.
I have not tried to copy custom shortcuts, how would I do that?
I have figured out that I can re-assign the keyboard shortcuts for the Object Menu - Fitting commands that do not work. However, I cannot re-assign them to the same command, I must choose new commands and those new commands work. So for example, if I delete the default command and assign Ctrl+Alt+E to 'Fit Content to Frame', it doesn't work. However, if I assign a different command, it works. Also, I can press commands to make them appear in the New Shortcut box, however, pressing 'Ctrl+Alt+C' does not work - nothing appears in the New Shortcut box. Although each of them alone, and any two of them combined works, just not all three together. Weird.
Shortcut sets are saved in your user profile, in the Keyboard Shortcuts folder, so you can copy them and use them on another computer if you like.
Yes, we tried this - I copied in another set of shortcuts from someone who had a full working set. It doesn't work on my machine. No idea why.
I did discover that Ctrl+Alt+C and Ctrl+Alt+E do not work in either Illustrator or Photoshop either. (Not sure if these are the only two, they are just the two I have been testing. Everything else I use regularly and have tried works.)
Interestingly, Ctrl+Alt+E works in Word, but not Ctrl+Alt+C
As I mentioned, it's not a good idea to try to use shortcut sets from an earlier version in CS6, so keep that in mind. Are you sure the OS hasn't hijacked this shorcut for something else, language change, perhaps?
I used a set from Indesign CS6, so it should be ok.
Not sure on the OS, I can't find anything, but I'm not entirely sure where to look.
Nor am I, but we're running out of other possibilities and it's getting pretty likely that SOMETHING else is using those shortcuts, either the OS or some other program.
Found it!! Ctrl+Alt+C is controlled by Catalyst Control Center.
-Right click on desktop background.
-select catalyst control center at top of list
-click on preferences
-select hotkeys at top of list
-select the one thing on the list, then press Edit
-change from Ctrl+Alt+C to Ctrl+Alt+Shift+C
-ok, ok, ok
And now it all works!!
Ah yes, Catalyst. Great video cards, lousy (and intrusive) user interface...