I've just completed a clean install of Vista and mIRC.

These steps led to a hang on my first attempt.

1. In Sounds -> Agents, I checked "Private events" and "Other events."

2. //dcc chat $me

3. I then pasted the following into the dcc chat with myself:

Code:
The mIRC homepage provides the latest news on mIRC, general IRC information, and links to many other IRC-related websites and resources. Please click on one of the links to the left to find out more. We hope that you find this website useful, and your experience on IRC enjoyable! Have fun :-)
mIRC 6.21 has been released! (November 23rd 2006)
As always this new version of mIRC addresses many of the issues reported since the last release. A lot of time was spent tweaking features and fixing bugs. We worked on the installer, logfile handling, script error handling, as well as a handful of other things. Only a few new features and functions have been added, making this mainly a bugfix release. 

For general users the interface has been improved in small but useful ways: the display speed of text in window and listboxes is faster; a new Minimize/Close All menu item has been added to the Treebar folder menus; the IRC Servers dialog has been updated to use a modern and snazzy treeview; for USB drive users mIRC now supports a -portable command line switch which makes it avoid use of the registry; A "Lock Bars" options as been added to the toolbar/switchbar/treebar menus to prevent accidental changes in their position/size; several new sounds events have been added to the Sounds events dialog; and the Sounds dialog Enable/Disable sounds option now disables all sounds completely, for users who like to run mIRC in complete silence :-) Finally, the logging feature has been improved to cope with multiple server connections.

For experienced users and scripting enthousiasts this new version has heaps of fixes, changes and other little thingies to play with. We encourage you to explore the whatsnew.txt on the Web to learn about the complete list of changes and their impact. Some changes are obvious, some need getting used to - please take your time to play with them and see how they work?

May we invite you to visit the Message Board on the website with all questions you might have? The board offers great help with everything related to mIRC!

What is IRC?
IRC (Internet Relay Chat) is a virtual meeting place where people from all over the world can meet and talk; you'll find the whole diversity of human interests, ideas, and issues here, and you'll be able to participate in group discussions on one of the many thousands of IRC channels, on hundreds of IRC networks, or just talk in private to family or friends, wherever they are in the world.    What is mIRC?
To use IRC you need a small program like mIRC, an IRC client for Windows, written by Khaled Mardam-Bey. mIRC is a friendly IRC client that is well equipped with options and tools. mIRC is shareware, which means that you can download mIRC and try it out freely for 30 days to evaluate it. If during, or at the end of, the evaluation period you decide that you would like to continue using mIRC, you must register your copy.


The hang continued until I killed AgentSvr.exe through taskmgr.

I just tested this again by pasting the following text:

Code:
Hello1
Hello2
Hello3
Hello4
Hello5
Hello6
Hello7
Hello8
Hello9
Hello10
Hello11
Hello12
Hello13
Hello14


The agent is now hanging after speaking "Hello11". mIRC isn't hanging though. When I continue typing more text to myself through dcc chat, the agent does not respond in any way.

Using this alias, I experienced the agent-only freeze again:

Code:
alias FreezeAgent {

  var %FreezeAgent.dec = 30

  While (%FreezeAgent.dec) {

    say Hello %FreezeAgent.dec

    dec %FreezeAgent.dec
  }
}


The agent only spoke up to the third line "Hello 28" before hanging.