Is that what's listed as the "Path and Filename"? If so, that should be the file that Sophos thinks is infected. That file is created whenever Safari is updated, and contains the previous version of Safari. It's a perfectly normal file.
Here's my guess as to what happened: Presumably, you must have been infected at some point, and must have given your admin account password to the malware to allow it to install itself inside of Safari. (Without the admin password, it couldn't install itself there.) Then, at some point, you updated Safari, and the infected version got archived in that file. Your current version must be clean, or Sophos would have found that.
So, though there is probably an infected file inside that archive, it's inert at this point and not affecting you at all. However, malware isn't a good thing to leave lying around. I've never messed around with that file, but you could try deleting it. My guess is that it shouldn't make a difference, but that's just a guess, so here's what I recommend. First, in the Terminal, paste in the following command (do not try to retype it!):
sudo mv /Library/Application\ Support/Apple/.SafariArchive.tar.gz ~/Desktop/SafariArchive.tar.gz
After pasting that command, hit return. When asked for your password, enter it and press return, being aware that nothing will be displayed when you type. (Note that you must be using an admin account when doing this.)
This will move the archive from where it is onto your desktop, as well as renaming it to make it become visible. (The period at the beginning of the name means it will be hidden in the Finder.) It can't do you any harm there. Now test for a while (a day or so), and if your system seems to be working fine, you can safely delete that file.
If it becomes necessary to put it back, just do this:
sudo mv ~/Desktop/SafariArchive.tar.gz /Library/Application\ Support/Apple/.SafariArchive.tar.gz
Since I don't honestly know what effect removing that file might have - though I doubt it will have any, I don't know that for sure - it would be okay to leave it there, and it should be replaced with a clean version of Safari the next time there's an update.
Edit: Could you e-mail me privately? I'd like to get a sample of that file for testing. You'll find my address here:
http://www.reedcorner.net/contact/