How do you load MIRC on another computer. I have looked in this forum and haven't seen anything pertaining this.

Just install it like you did on your other computer. If you registered it, just enter your registration on the new computer.

say I've got three computers, and I put mirc on them, and register with the same key on all three.

then I want to replace one of those computers; and intend to remove mIRC from the one being replaced.

am I going to have to buy another license key for the replacement machine, even though the number of machines using that key is not, in theory, exceeding three, the maximum number of machines a key may be used on?

I would surmise that this could also apply to having to format a machine's HDD and reinstall everything.

You can use the key on 3 computers at the same time for your own personal use. As long as you don't exceed that, one key is fine.

worry not, it's my own use. I use multiple machines to run stuff with varying hardware expectations, but mIRC ended up on all of them.

well. that's good to know. having to pony up again when I get to the "flatten and reinstall" point with any of the machines would kind of, well, bite. such is life on a tight budget. *le sigh*