Since we now have a subforum for Electrum, I am rewriting here what I already said in other threads. I hope it's more visible in its own thread.

It is not safe to use a portable version of Electrum on an insecure computer!

Don't get me wrong: I am not saying that a portable build is by itself more dangerous than a non-portable version.
However, a portable version does not bring anything more in terms of security. It does not protect you from the computer you are using.
In addition, portable builds encourage dangerous behaviour, because they make it very easy to use your wallet on third party computers, that might be infected with viruses and keyloggers.

I was never enthusiastic about distributing portable versions of Electrum.
I did it because the demand for portable versions was so high that portable builds distributed by third parties were getting popular.
That's the only reason why I accepted to distribute portable builds: I do this in order to avoid an even worse situation.