Ahh. This happened to my parents. What happened is that they got a trojan, and so they had a copy of mirc running that was hidden from the task manager in all ways. So all that would happen is that mirc would beep the speaker on certian events and it was really annoying.
Go out and get yourself a virus scanner, there is a free one that does a decent job at
http://free.grisoft.com (for personal use only).
But if there was anything, that would be my bet.