Your browser has been hijacked. Some spyware has been loaded on to your computer, too, I'll bet. This has happened to me. The cure is probably in a registry hack. First, load Spybot S&D and run that, but make sure you update your definitions file before you run it. Also do a search on the name of the site that has taken over your browser. Don't go to the site, though. Instead, look for forums and removal tools. If you go to the offending site again, it will reload itself on your computer. Xupiter is one of those browser hijackers that won't go away. Spybot can get rid of it. The problem is that they divert your browsing thru their server, and steal your credit card numbers, passwords, etc. I would not use your computer for much of anything until you get rid of that problem. And remember to be careful that, when you search for the solution, you don't get fooled into going back into their site. They are tricky. Usually, their program will go back out each time you reboot and reload itself. It disperses itself onto your hard drive with pointers between each copy, so that you have a very hard time deleting it. It will most assuredly require a registry hack. Try this site for starters:
http://www.spywareinfo.com/. There is a lot of good info there.