The main download webpage of Ubuntu Desktop allows you to download the ISO files using HTTP. I have found that downloading these large files using the browser is really slow and sometimes leaves them incomplete or corrupted.
Instead, I find that using BitTorrent to download these ISO files is not only faster, but also error-free. The BitTorrent files for Ubuntu ISO can be found at this alternate downloads page. Use your favorite Torrent client to download Ubuntu and install it.
Tried with: Ubuntu 12.04.2 LTS