You can reboot while holding down the Command-R keys to boot into the Mountain Lion Recovery Partition and from there reintall a fresh copy of Mountain Lion from the Internet for free. From there you can also use its Disk Utility on your Macintosh HD to first "Repair Drive", in case that simply fixes it. And you can also run "Repair Permissions" from there.
Oh, and please Update your system Info in your Forum Profile. It says you are running Tiger 10.4
Hope this helps