To state the obvious, which is seems like you already know, you're seeing this error because you're missing the class definition files for Mage_Archive_Tar.
What i've noticed when checking further on the problem is that my Tar.php file get's removed, when trying to ftp it back up again in the right directory it just gets removed once again after trying to install a extension once again. Anyone who knows what thous problem could be because of ?
There's a few things that come to mind here.
Your hosting provider (you didn't mention who they are) may have some sort of security audit software running that's removing the file
You have a deployment setup where there's actually multiple front-end web servers, but you're not using a shared NFS mount between them. This means you upload Tar.php to one server, but since it's not on the others your still fail, and FTP is connecting in some sort of round robin way
Your system has some sort of continuos deployment running that's moving files from place X (a source control repository or staging server) to place Y (your production server), and removing any changes.
I'd poke around your Unix cron jobs, as well as any custom Magento cron jobs installed by extensions. It's possible (although not likely) that something in there is deleting the file on a regular basis as well. I'd also try putting different files in the same folder and seeing if they vanish to narrow things down.
Good luck!
