/dev/sda1 contains a file sytem with errors!
(for people who don't got any pc-skills)
Start the system, put the Ubuntu cd in the drive, restart. Make sure you have selected the cd-drive as primary boot device.
Select the "try Ubuntu without any change" and let it run.
In Ubuntu click;
sudo fsck /dev/sda1
(sda1 could be: sda2 or sda 3, etc etc. see your error message)
The system will start running a scan. If any errors are found, press y to "ignore error?" and y to "force rewrite".
You can put some weight on the "y-button" and do something else, since you don't want to press 100 times the y for about 20 minutes
It will do some 5 pass tests.
If you got a message;
FILE SYSTEM WAS MODIFIED
"xx/xxxxx files, x% non contigious, xx/xxxxx blocks"
You can close the window, take your cd-rom back and restart the system.
Now everything would work again.
In the Terminal, you could use some other commands also, but didn't test it since the first one worked for me.
sudo unmount /dev/sda1
sudo fsck -y /dev/sda1