Many folks in EFnet #mIRC run v6.03 & Win XP without complaints. Maybe we can tack this down a little bit more. Is it possible for you to run mIRC only .. say over night or for an extended period of time to see if anything else is causing it ? Also, might want to check your windows start up section, and ctrl + Alt + delete to be sure you know what's running on your machine.
I know you said this is a new copy of mIRC, from mIRC.com, but you might consider getting/running a virus detection program. Just to be absolutely sure.
