How is the /timer command in mIRC implemented? Does it use a Windows API call, or does it use a low level routine? I'm trying to figure out a way to make a function in C similar to the /timer command...

Use the timeSetEvent (/timer) and timeKillEvent (/timer off) API functions.

You also need a timeproc function (Much like window or thread procedures) for the timer to call when it fires. Look on MSDN for more info.