Ubuntu Freeze

I was tried to open 'About' page on Ubuntu Linux. System> About Ubuntu. But failed. Look at the images below. Desktop goes completely hang. Mouse, keyboard stop/ not responding to my commands.

This is first snapshot of freeze Ubuntu desktop

It is a snapshot of hanged 'About' page on Ubuntu Linux.

My Ubuntu 10.04 system behave like a frozen bear. I waited for up to 5 minutes, but nothing happened. Ubuntu did not come back to normal stat. So at last I restart computer pressing restart button on body/ computer casing. I know that it going to hang is a rare situation in Ubuntu. I also facing this kind of freeze or not responding type problem in Ubuntu Linux 3rd time. At first time I faced going hanged problem when I tried to install a program. 2nd time I thought Ubuntu crashed or melt down.