Happens sometimes for me (Win XP Pro SP2)... but the way i see it is:
If i kill explorer.exe process it restarts and everything is normal.
If explorer.exe crashes some icons are gone, mIRC is one of them (another program is peerguardian - the icon reappears on tray but I can't click on it.)
My guess is that this is program issue (so it should fall under bugreports imho)
As for mIRC - if it happens I start another instance of mIRC and enter following:
//dde $ddename Command "" /showmirc -r
Just be sure you have DDE enabled in mIRC options.