I've been using mIRC for many years, always upgrading it to the latest edition, but the recent upgrade caused a problem. Now, when I want to send a private message to one person using the /notice option, the message is on display to everybody in the Main room. It's never done this before.
Is there an available script I could use for private notices to override a possible problem in mIRC?

One way this happens is if you have a script that has intercepted the /notice command using an alias of the same name, and does something different with it than it should. If you disable scripts with:

/remote off

and it stops happening, then the problem is caused somewhere in one of your scripts. Be sure to turn back on with:

/remote on

You can hunt for the offending alias with:

//echo -a x $isalias(notice).fname

If "/remote off" stopped it from happening, but there's no alias named notice causing this, there may be an event intercepting the INPUT event and using "msg" to display what you type in the editbox, even though they're supposed to treat input differently if they begin with /

If neither of these is happening, it's possible that you're at a new network that's causing it. The 'notice' command depends on the irc server to handle it correctly, and I wasn't aware of any irc servers not recognizing it until someone using Twitch said that 'notice' gave an invalid command error. It's possible for a server to handle incorrectly by sending it the same way as done by /msg

The /msg command and the /notice command both directly interface to the IRC server, and does not perform actual message handling. The audience that sees your messages is determined by the target you specified, and ultimately, any wacky designs of the irc network you are on.

Usage: /msg Target Message goes here.
Does: PRIVMSG Target :Message goes here.

Usage: /notice Target Message goes here.
Does: NOTICE Target :Message goes here.

Nothing about this simple construct has changed in any version of mIRC.

Thanks guys.
When I upgraded this time, I failed to keep my previous settings which meant I had to re-load some scripts into the Editor.

It seems that this is where my problem now is.
Thanks for pointing this out to me.