Ubuntu 8.04 reached end of life a while ago and is no longer supported: https://wiki.ubuntu.com/Releases
Your options are:
1. Do an upgrade to 10.04 LTS. See this: https://help.ubuntu.com/community/Lu...%2010.04%20LTS
You can do an upgrade directly from one LTS version to the next.
2. Or do what I prefer. Do a clean install of either 10.04 LTS or the newest version 11.04.
Ubuntu 11.04 will have the newest stuff. Ubuntu 10.04 LTS will be supported until April 2013 for the desktop version. Ubuntu 11.04, like all non-LTS versions of Ubuntu, are supported for 18 months.
Write back if you need more help.
And welcome to the LQ fourms!