The only small problem I had, which WASN'T related to mIRC (that I'm aware of) was I had to change the permissions for the mIRC subdirectory from 'read-only' to full permissions to allow mIRC to write to the files in that directory. Once I resolved that issue, it's been great.
This is a known problem with Windows 7. By default, Windows 7 does not allow writing to directories made under c:\Program Files.
As you stated, changing the permissions is an option. Additionally, installing to a main directory other than c:\Program Files or setting your Windows Login to use Administration configuration will also work.