Scroll down to the bottom of this page:

home.nc.rr.com/sarle/HairLinks.html

There are two sections of links on computer security.

First, as a preventive measure, install SpywareBlaster (free) and read twebman's page on security settings.

Then, if you haven't already, install and run Adaware and Spybot (both free).

If you still have problems, pay for the Cleaner, which is good for both removing and preventing trojans.

If that still doesn't fix it, install and run HijackThis, post the output at one of the security forums, and ask for help.