You can just move the files to another location. You don't have to re-install. Just keep in mind that mIRC can't uninstall itself from Uninstall Programs if you move it. You can still manually remove the files, but it won't do it for you.
To keep everything together, place mIRC.ini into the same folder that mIRC.exe is located in and install somewhere other than Program Files. Also, make sure that wherever you install it, that you have full permission to read and write to that folder. Depending on your security settings, even something like C:\mIRC\ may not work well, though it normally will be fine there.