As others have mentioned, if someone has enough access to your computer to read your servers.ini file, they can probably do anything else, such as install monitoring software. That said, I can understand why you are asking for this.
Most applications store important data, such as passwords, using encryption or obfuscation that can be easily reversed. This usually serves as a minor deterrent to a casual browser but it would not hinder someone who is intentionally looking for the data.
A more secure method would be to request a master password on startup that can then be used as the encryption/decryption key. mIRC already has an Options/Lock dialog with an "on startup" option. I should be able to extend that feature so that the lock password is used as the encryption/decryption key. This has been added to my to-do list.