Someone please Help!

Ok, so I visited a site that I guess I shouldnt have and now I have a serious virus. But, it is only under the "administrator log in". I am still able to log in under guest and everything is normal. As soon as I log in under admin, the windows start up sound comes on, then a womans voice telling me that my computer is under threat, and so on, its hard to understand. I cant access anything. No start menu, nothing on my desktop, I dont know what to do. I work from home and I need this stupid thing fixed, usually I would know what to do, but I'm clueless on this one. Can anyone help me?