/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;
Applications-Accessoires-Terminal
then type;
sudo fsck /dev/sda1
(sda1 could be: sda2 or sda 3, etc etc. see your error message)
press enter
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.
edit;
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
then
sudo fsck -y /dev/sda1