Ensure all programs using mIRC directory files are closed.
Navigate to the directory that contains your mIRC directory. Right-click on the mIRC directory, choose Properties, choose Security tab, click Edit...
If you aren't worried about security, choose the "Users" group and enable "Full Control".
If you are worried about security, click the Add.. button, type your windows logon name in the text area, click OK. Select your login name from the list and enable "Full Control".
Click OK to exit all dialogs.