If you want a clean install, the latest installer has a switch for silent installs.
mirc62.exe /S
Otherwise just sfx the whole folder and create shortcuts
Then import these two registry values with regedit /S mirc.reg
CODE
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\mirc\License]
@="XXXX-XXXXXX"
[HKEY_CURRENT_USER\Software\mirc\UserName]
@="XXXXXXXX"
All other reg entries, including uninstall, will be made first time mirc is run.