As clarification, when you run Invision from its link, it will ONLY search for mIRC if it's not in the same folder. Just like if you move a program and try using the shortcut later. The Invision link (start menu or desktop) is ONLY a link to mIRC. Invision isn't a program. The link must match the location of mIRC or it will not work unless it can find the right location.
You can find me at #Invision on irc.irchighway.net or #OldGames on irc.undernet.org if you need me for anything.