[ubuntu-za] Upgrade to Ubuntu 9.04 error

[Dr. Johan P. Prinsloo]

I'm trying to upgrade my one PC to 9.04. All 3 the other PC's worked
fine and went through the upgrade without a glitch, but this particular
one, which is my own PC keeps giving the following error;

Error during update
A problem occurred during the update. This is usually some sort of
network problem, please check your network connection and retry.

W:Failed to fetch
http://security.ubuntu.com/ubuntu/dists/jaunty-security/main/binary-i386/Packages.bz2 
Hash Sum mismatch , E:Some index files failed to download, they have
been ignored, or old ones used instead.

Any advice?

Johan