Failed to fetch Ubuntu repository: error is "404 not found".
 

Failed to fetch http://us.archive.ubuntu.com/ubuntu/...-i386/packages 404 not found

how can i fix this?

What version of Ubuntu are you using?
The link that you gave is for Ubuntu "Jaunty Jackalope", which was Ubuntu 9.04. The 9.04 version of Ubuntu reached end of life on October 23, 2010. See this: https://wiki.ubuntu.com/Releases When a version of Ubuntu reaches end of life it's repositories go offline since it is no longer supported. This is why you get the 404 error.
The regular versions of Ubuntu are supported for 18 months after they are released. The long term support (LTS) versions are supported for 3-5 years for the desktop versions.

As for how to fix this, if you are using Ubuntu 9.04, you should do a clean install of either Ubuntu 12.10, which is the current (non-LTS) version of Ubuntu and has all the newest kernels, drivers, apps, and features. Or you could do a clean install of Ubuntu 12.04, which is the current LTS version and will have a much longer support lifespan.
If you are using a version of Ubuntu other than 9.04, and you have decided to add that 9.04 repo to your system for some reason, you should remove it. Mixing repositories from different versions of Ubuntu is always a bad idea. I have seen people around here do this, and it always ends badly.

Write back if you need more help.
And welcome to the LQ forums!

If you ARE still using 9.04, you could manually change your repositories to 12.04 and attempt and apt-get update && apt-get upgrade, reboot, apt-get dist-upgrade just for S's & Giggles. Good chance it would fail miserably and explode. But might be fun to watch.