It's a good practice to verify that your Ubuntu downloaded cd's are correct, for that reason check the hashes published by Ubuntu in the following page:
https://help.ubuntu.com/community/UbuntuHashes
You can find, a lot of md5 free checkers for windows in:
http://download.cnet.com/....
In linux is more easy to find. Here in the documentation of ubuntu is a good description:
https://help.ubuntu.com/community/HowToMD5SUM
Or try installing the package directly:
sudo apt-get install md5sum
