Help computer virus !!!!?

So something came up saying my computer is infected by a Trojan. All I did was go on myspace and use an app called bumper stickers. I have used it plenty of times. But this time something bad happened. I'm not even sure it's is from this website. I have no clue what to do I can't get online except to me credit card companys which scares me. I couldn't even shut my computer off. Help please! Can having a virus or something allow someone to use my credit card numbers or rake pwords from my computer? Does turining my computer off make it so nothing else can happen?