I have %nlistpath set to my $scriptdir path. I have this in my dialog:
icon 2, 9 9 15 15, " $+ %nlistpath $+ icons\ $+ op.ico $+ ", 0

The first time I open the dialog, it works fine (showing the icon), but the next time I open the dialog I get problems. For some reason the icon line above writes to the %nlistpath and screws everything up, so what I'm wondering is how I can stop this from happening.

Thanks in advance.

Off the help file:

Variables
If you specify a %variable name in a dialog item definition, the %variable will be set with the contents of that item when the dialog is closed.

edit "", 2, 10 10 100 20, autohs %result

Maybe this is what's wrong. mirc thinks that %nlistpath is there for the above purpose. Try
$+(",%nlistpath,icons\op.ico")
or, if that doesn't work either, make a custom identifier like and put $nlistpath in the icon item.

yes ive also had a similar type problem in my attempt at a nicklist ...... i had to set an alias to actually read the information from what i wanted just like how qwerty is explaining in his example ..... mine was an attempt to set font color thru mdx urs is i think your attempt to set icons to the nicklist thru nicklist.dll ( btw i hope u get more luck with yours than ive had with mine .. i wanted to have mine have separate icons for away users which the nicklist dll doesnt allow .... grrr ) if your interested i do have a working example of a nicklist someone gave me to check out so i could learn alil more about nicklist dll ..... id gladly send it your way so u could look thru it and see how they got thru all of the settings your trying to do now just pm me or something with your email addy and ill send it to u to look at if your interested