Create the following directory layouts first:
\$OEM$\$Docs\All Users\Start Menu\Programs\mIRC\
Install mIRC as you normally would, or if you already have a copy installed - browse to its folder and copy all the files, including its sub-folders (if any) to \$Progs\mIRC\
Copy the 'Shortcut to mIRC' from your start menu and place it into \$Docs\All Users\Start Menu\Programs\mIRC\
That's all you need to do! If Windows XP is installed to D:\ (for example) Windows Setup will copy your mIRC files to D:\Program Files\mIRC, and your shortcut to D:\Documents and Settings\All Users\Start Menu\Programs\mIRC.
» Registering mIRC
You can import this registry key from either cmdlines.txt or [GuiRunOnce]:
Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\Software\mIRC\License] @="xxxx-xxxxxx" [HKEY_CURRENT_USER\Software\mIRC\UserName] @="MyName"