I Got A Trojan Horse!

I picked up a terrible virus on my computer -- a Trojan Horse called Trojan Vundo! I have never had this happen to me before, but it is horrible. It has attacked my computer and slowed it down to the point where it's almost completely dead. I don't know how I got it and it's going to cost me some money to get rid of it. What a hassle! Why do people create these Trojan Horses? What does it achieve? I hate it!