@TECO - Sorry, but this suggestion is just plain wrong. The reason that programs and data are separated in Windows is for security. You do not want a malicious program to be able to add a virus to executables, so executables are placed in a "Program Files (x86)" directory that requires administrator permissions to write to, preventing a virus that isn't running as an administrator from infecting it.
If you use the old-style installation to have executables like mirc.exe and data files like mirc.ini in the same directory, then you are increasing the security risk on your PC.
In short, this is a very bad idea unless you know exactly what you are doing, have an essential reason for doing so and fully understand the risks you are taking.
P.S. That said, does a portable installation give you what you want?