Ubuntu 11.04 is Natty Narwhal is no longer supported in any way. The repositories are not there any more.
The repositories still exist but have been moved and are no longer being updated.
If you want to continue using an outdated release then edit /etc/apt/sources.list and change archive.ubuntu.com to old-releases.ubuntu.com
I would not recommend this as there will be no more bug fixes, security patches etc
I strongly suggest you upgrade to 12.04 LTS. This will be easiest as a clean install. The alternative is to upgrade to 11.10 and then upgrade again.
I suggest you try a live CD/DVD of both Xubuntu and Ubuntu without installing as the desktop has changed significantly: Xubuntu is more like Ubuntu was in 10.04 and Ubuntu now uses Unity and this may be too slow on older hardware. Once you have decided which you prefer install it.
Downloads are here:
Ubuntu -- http://releases.ubuntu.com/precise/
Xubuntu -- http://xubuntu.org/getxubuntu/
You can of course go with 12.10 but not being an LTS release you will need to upgrade sooner.