This was a change in version 6.3 for Windows Vista compatibility. However, if you install mIRC someplace other than Program Files or Program Files (x86), settings can be stored in the install folder. To do this...

1) Create a new folder in the desired location and name it mIRC. ex; C:\mIRC
2) Create a txt file inside the new folder and name the file mirc.ini
3) Install mIRC to the newly created folder.

From: versions.txt
16/08/2007 - mIRC v6.3

Changes:
..
20.For Vista compatibility, new installations of mIRC now save all settings in the User's "Application Data\mIRC" folder by default.

However, if the installer or mIRC find mirc.ini in the same folder as mirc.exe they will use the mirc.exe folder for all settings. Users installing over an old version of mIRC should therefore see no change in behavior.

The installer is now also aware of administrator and standard user accounts and installs mIRC accordingly.