I downloaded backtrack 5 r3 by torrent and checked the md5sum, which was OK. I installed BT as a virtual machine and ran
apt-get update
, then
apt-get upgrade
. During the upgrade, the connection was terminated by Eset Smart Security, as a Trojan was connected: JS/TrojanDownloader.Agent.GJ trojan

gem install librex -v '0.0.68'
,which is where the trojan was detected in the upgrade, causes Eset to interrupt again with the same problem.

What should I do?

I was surprised by this. I don't think I would have detected this had I been installing BT onto bare metal.