[mbraudel]

It sounds like you've been infected with a hijacker.

I got hit with one about a week ago. It did what you describe, and also brought with it a couple of viruses.

It came in despite me having a firewall and antivirus software running.

In order to detect it, I downloaded Microsoft's free anti-spyware utility.

There's another free anti-spyware utility available from Yahoo, but that one didn't catch the hijacker that infected my PC.

For me, the solution was:

Run the Microsoft utility to detect and delete the hijacker.

Run a full system sweep with my antivirus software to get rid of the viruses that the hijacker had brought down.

Then look around on my desktop, and in my browser favorites, and in my Start/Programs menu, to find all the places where the hijacker had embedded shortcuts to places it wanted to send me.

I can't be sure that you have the same problem I did, but it sounds like a hijacker to me.