So as noted in Sathya's comment, he is running under Vmware.
The reason this doesn't work is because VMware Player is taking control of Ctrl+Alt, because that's the key combination it uses to "break out" of the VM.
There are two ways you can fix this:
There is an escape override built into VMware which overrides the breakout combination, by tapping Space while holding down the breakout combination, before hitting your desired key So for example, if you wanted to go to the right workspace you would hold down Ctrl+Alt, tap Space and then tap Right.
Change the Combination
Change the breakout combination. To do this, edit preferences.ini (in Windows 7 it's in
C:\Users\$YOUR_USERNAME$\AppData\Roaming\VMware, if you can't find it search) and add:
pref.hotkey.shift = "true"
pref.hotkey.alt = "true"
pref.hotkey.control = "true"
This remaps the breakout combination to Ctrl+Alt+Shift.
Before you say anything, yes, I realise that this is also used in Ubuntu, to move windows between workspaces. Unfortunately, that is the best I've been able to come up with so far. You are supposed to be able to be as specific as left or right control, but they don't work for me, leftControl etc just maps back to both.