I have downloaded a trial version of cs6 and for some reason i can't see the marching ants when i make a selection, i can use the marquee tools and draw the selection, and the marching ants are there,but as soon as i let go of the mouse the ants dissapear, i have tried clicking and unclicking the selection edges in view but that makes no difference.
Could you please post a screenshot with the Options Bar visible?
Could you have set the Tool to »Subtract from Selection«?
You have set the Tool to »New Selection«, so I don’t know what the issue is.
Just to make sure: After you have dragged out the Marquee is Select > Deselect grayed out?
In case of mysterious issues you could always try trashing prefs by pressing command-alt-shift/ctrl-alt-shift while starting Photoshop until the appropriate dialog appears after making sure all customized presets like Actions, Patterns, Brushes etc. have been saved and making a note of the Preferences you’ve changed.