The list of possible problems is almost infinite.

When you installed mIRC did you go with all of the default settings, or did you change some of them in order to maintain scripts, channels list, servers list, etc.?

Were there any problems encountered while downloading or installing mIRC?

What OS are you using? How much ram, available hard drive space?

Does mIRC show up in your task bar, as an icon by the clock display, both, or none of the above?

Let's get those questions answered, then see if we can narrow down the exact problem.