Disable alt-tab (option-tab) instead of command-tab I am using Mac OS X 10.8.4 on a Macbook Pro and somehow alt-tab is switching between the applications from Dock. I would like alt-tab to work within RDP (Windows) only.

I don't know how (or if) did I enable that (i do not have KeyRemap4Mac or anything like that).

Is alt(option)-tab supposed to work as default?

Any ideas how I can disable this behaviour?
It sounds like you've managed to change your keyboard mappings.  Assuming you haven't installed 3rd party mapping software, the quickest fix is to restore the mappings back to default.

SystemPreferences -> Keyboard -> Keyboard Shortcuts -> Keyboard &amp; Text Input -> Restore Defaults

Most likely one of the "Move focus to X" options is set to alt-tab