I agree with the previous posts that your problem could be due to another program. It's possible and probable that another program you're running may act weird when mIRC is running, specifically A/V programs but potentially others. Without actually looking at your cpu usage and seeing mIRC at 100% it's really hard to say that mIRC is at fault here. I've never heard of an issue where a vanilla mIRC caused this kind of problem. You really should be able to hit ctrl-alt-delete responsively even when your computer is running slow. If not, run taskmgr.exe *before* starting mIRC, keep it in the foreground and watch your *processes* window.
On a sidenote, do you have reload logs on with many channels and a lot of logfiles?
