I can say that the most recent version as it stands of mIRC seems to run fine under Windows Vista Beta 2 as Standard User (no admin rights, the default). Far more so than any other client I've tried.
The reason I'm posting in bug reports though is that there are two issues, one slightly more serious than the other.
Firstly, Microsoft have dropped support for WinHelp and as such, the help files for mIRC no longer work at all. Opening help in mIRC will open the Windows Help and Support Center, explaining the lack of compatibility.
As such, one key thing that will need to be fixed between now and next year is a recompile of the help documents from WinHelp (hlp) to HTMLHelp format (chm). Microsoft have a nice free chm builder application at MSDN to create chm files and hopefully make writing the new help relatively painless.
The other less critical issue is possibly the fact that while mIRC works, it does so only thanks to Vista's compatibility wrapper. That is, mIRC thinks its saving its configuration files to C:\Program Files\mIRC when in fact they are being quietly written to and then read from C:\Users\#username#\AppData\Local\VirtualStore\Program Files\mIRC. Obviously to support multi-user PC's and improve compatibility, mIRC should start to move to storing preferences over in the correct User's appdata folder rather than Program Files where you can now only write to if you have administrator permissions.
Just pointing these out and hope these things can be fixed in plenty of time by next year. ^_^
