Upgrade of BackTrack 5 r3 downloads Trojan

I downloaded backtrack 5 r3 by torrent and checked the md5sum, which was OK. I installed BT as a virtual machine and ran

Code:

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apt-get update

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, then

Code:

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apt-get upgrade

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. During the upgrade, the connection was terminated by Eset Smart Security, as a Trojan was connected: JS/TrojanDownloader.Agent.GJ trojan

Running

Code:

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gem install librex -v '0.0.68'

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,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.