Weve blocked the threat HTML:ExtortMainNT-BOT[Scam} from harming your system

I get this message everytime I start my computer. Normally you can search online and find articles on such names of viruses.
There isn’t a single mention.
Is this legitimate or a way to make me buy the full version?

There is little information to work with in your post.

The malware name in itself isn’t that useful, A screenshot of the Avast Alert window with the See details option selected might help.
Attaching Images to your post - When you Click the Reply button it opens a text window for you to post your comment (reply or post).
Click the Preview button, that shows what you have input and expands it to include ‘Attachments and other options’. Click that it further expands, here you can attach images, etc. at the bottom of your post.
See my attached image, click to expand.

Since this happens when you start your computer (and you aren’t making this connection), something on your computer is trying to make the connection and Avast is blocking access to a site which it considers suspect at the least.

Thanks for that. Please see attachment as requested.
Normally you are given ways to remove a threat with AV software. In this case I’m repeatedly told about it without any assistance offered.
This is why it appears to be a way to get me to buy their firewall version.

I don’t see a screenshot attached to your reply.

If it is blocking an attempt to connect to a website which an avast shield has blocked (typically that would be the Web Shield). There is nothing to remove as such, what needs to be pinned down is what is making this connection.

Even Avast Free (which I use) has a Firewall component See attached screenshot.
So I’m not sure what version you are referring to or what Avast Program version you have (I suspect Avast Antivirus Free for Windows) ?

It’s called Avast Free Antivirus. As you say, its probably the web component nagging me. It’s stopped now thanks.

You’re welcome.