1. Are you using a 64 bit machine? Is the machine powered by an AMD processor? (Intel doesn't make processors for 64/32 installs). If not them I'm surprised that even Windows 64 would work on it.
2. It might be that mIRC will not work on a 64 bit version of Windows in the same way that the 16 bit mIRC won't work on the 32 bit version of Windows.

I suppose Khaled will need to aquire a 64 bit Windows before being able to get mIRC running smoothly on it. Then again, with the slackness (I know this will offend some but stiff bikkies) of updates over the last 18 months I'd be surprised if Khaled even cared if mIRC worked in more modern environments.