why my computer consider MQL5 a virus Win32/Themida ?

Probably a false positive.

I think you're right, but that it's a special kind of false positive. MT5 has clearly started using the Themida code protection from http://www.oreans.com/. Antivirus software such as AVG is flagging this up as a problem because the code is protected... and therefore can't be analysed by antivirus software. Without this check, virus writers could evade detection by wrapping up their code in something like Themida.

This is a big problem. Personally, I'm not sufficiently interested in MT5 yet to bother overriding AVG; I haven't updated since the last pre-Themida (or pre-warning) build. And if I can't be bothered, what percentage of "retail" users of MT5 will be put off by the big, scary antivirus messages?

Personally I use Avira and every false positive I ever had I sent them a sample and within a day or two the auto-update solved the problem. Wouldn't the same happen in this case?

Perhaps, or perhaps not. If AVG isn't capable of scanning "inside" something which is Themida-protected then, for safety, it's got to be flagged up.

Hmmm... I don't know that much about virus protection technology, but I imagine that if the anti-virus cannot scan "inside" the file, they would still have some kind of 'fingerprint' (hash or whatever) technology that would identify the file itself as not being a virus (after the specific file with the specific fingerprint was analyzed at the anti-virus company).

Sure, but they'd have to redo that check and that validation on each new build of the software. It's quite possible that something like that already happens with MT4, but it's quite a burden.

Well, I guess nothing to do about it but hope that there won't be a problem.

Themida has been around for sometime even in MT4, so it's not exactly unknown to the anti-virus software makers. So it doesn't make sense for them to flag it as a problem simply because they have no idea what's inside.

Anti-virus checkers have to work on the basis of what it knows to be a problem (or at least a strong likelyhood), and not based on what it doesn't know what's "inside". Simply because there's a lot more that it doesn't know than it does, which would make the software unusable.

So, what's your hypothesis for why AVG flags up this MT5 build? Are they just being dumb? Why would they block something like this if it's possible and practical to work only on a known-problem basis?

Not at all. There are things such as false positives and bugs. If they don't work on known problems (or very likely ones), they would have to flag everything they don't know as a "problem". Everytime you compile and run a new program, it would be flagged because the virus-checker software "doesn't know". That would make the software unusable.

And why do you think viruses are given specific names? Is it because they know specifically what it is and can identify it, or it is simply because they don't know? Here's something I found on Google, and I'm sure there's others.