Nope, it's not a virus and it's not spyware. I have the same problem of mIRC closing randomly on XP. I have the same version of mIRC installed on a Win98 system in the same home network (same router, same firewall, same DSL connection) and it doesn't have the problem, nor have I experienced this random closing before with Win95 or Win98 in the 7 years I've been using mIRC. It's gotta' be an XP thing...just seems that not enough people are experiencing it for anybody to have figured out the cause, much less a fix. *sigh*
Oh, yeah, one clue. On my system, mIRC only does this random closing down when I change focus from any program back to mIRC. It doesn't do it every time, of course, but changing focus is the only constant. It doesn't make any difference if I toggle between programs with the mouse or with alt/tab.