I was using Chrome as Browser. I tried using File manager to upload quickstart, extract and install. I tried using filezilla, deleting all files and extracting locally to FTP them. I even had Siteground try to perform themselves and finally they said the package must have problems, for they created 2 additional folders with 2 new Joomla installs on my server, one 2.5, on 3.0 and those they installed downloading from Joomla, worked fine.
The problem is the 3.0 quickstart. Did you try that one? Finally, I deleted all and installed the Joomla 2.5 quickstart with no problems at all. 3.0 was impossible to install, for it looped in installation 3rd step.