Have you checked for viruses?
You can do a free online scan with
Trendmicro. You can also download two very good (and highly recommended) trojan scanners. Both of them are free ->
The Cleaner and
SwatIT. A free virus scanner is available from
Grisoft called AVG.
If no trojans/viruses or any other infections are found, try reinstalling mIRC. Make sure you go to
Start > My Computer > Add/Remove programs and uninstall mIRC first. Before you redownload/install you might want to check if mIRC still starts up by doing a reboot.
You could also visit
Microsoft's Support page.
G'luck!
Regards,