Is it possible that while you were out of the house, he could of booted up your computer and put some sort of Trojan on your computer (ie:Sub7, NetBus, Back Orifice).

If he boots up your computer and presses F8 after the bios screen, and goes to the command prompt, he could of installed the Trojan this way.

And you said that you are sharing a Dial up Internet Connection, Do you have file and Print Sharing enabled?
If yes then disable this immediatly!

And are you using Internet Connection Sharing?.
[i]I mean how are the computer set up to reach the phone line.
Are your computers set up so that the you have a phone cable going from 1 computer to the other, then to the phone line??
Or do you each use a seperate phone line?

These are the questions that are running through my mind, but i'm sure that others will jump in soon with more questions.