Help with desktop background

Hey, all. I don't know if you can help or not. I'm trying to help my parents, who have a Dell. There is something stuck or wrong with their Desktop Background. It started out as some animated thing that offered Comcast ads, too. Looked like a virus of some sort to me, and someone in another forum suggested their background was set to a Web page.

They were right that it had to do with the background (I was looking for some sort of file, and I couldn't find one). But when I go under properties to change it, it doesn't let me scroll through the backgrounds and choose one. I'm stuck with the one that's on there. I was able to go to a site and download a free wallpaper. That wallpaper appeared, but I still can't go in and change the desktop appearance. I'm wondering if that initial odd thing WAS a virus of some sort, and maybe it's still running underneath the wallpaper I just downloaded.

I'm very confused. Any suggestions? Does any of this make sense, or am I just rambling?