Did you have python launcher installed and then uninstall it?

I had this same issue a week or so ago, and it looks like when I uninstalled python launcher it didn't repair the link for "/usr/bin/python"

If you run the command
it will give you the target of the existing python link. it should point to "/usr/bin/python2.5", or "./python2.5" (the former being preferred)

If it shows a different target, you'll need to run the following command as root to get it working:
I hope this helps fix the issues you're seeing!