mikael - I have to disagree. This is an Avast problem. I haven't used my computer all day - the only thing that has changed is that Avast updated and I'm having the exact same problem as woodbri. I actually ran across this post searching for what it could be. I've had encounters (not on this computer or any of my own, but on a coworkers computer) with the Vundo virus, and it has never affected Avast like this.
In addition, there are no other symptoms of a computer virus. My computer is running quickly, there are no unknown .exe files when I check my running applications via Task Master, no other anti-spyware (I have Spybot, Malwarebytes, spyware blaster and spyware guard on my computer) are detecting anything. Vundo is a common, old virus - there is no way these programs would miss it.
Like woodbri, this ONLY started happening the moment that Avast was updated. For it to happen at the same time as woodbri, with the same problems, only after the update is telling.
Edit, since no one's replied yet: I just did three full system scans with malware bytes, avast and spybot search and destroy. Nothing was found. This leads me to suspect that this is either a problem with the two sidebar sites (one in my case, its the google weather site, as I don't have the other), or a problem with avast giving a false positive.