Well, I'm out of ideas... maybe someone else has an idea. A clean install of Invision/mIRC in a directory without spaces should work without any problems.
You might try downloading the file from that contains mIRC and Invision all together as a set rather than getting each separately. You can find that file by clicking on Invision at the top and then it will be on the left side about halfway down. Other than that, I don't know.