I'm not sure what to do next

Hi, I'm not to tech smart.

I went to play Counterstrike: Source and a message came up telling me that the game was not available right now, and to try again. At the same time, Norton told me that auto-protect had detected a trojan horse, and blocked it. However, I still can't play CS:S. I ran a Norton scan and it detected a cookie and a trojan. The trojan was in the 'resolved - no action' catagory, Norton asked me what to do with the cookie, I selected to remove it.

Thinking that the trojan would be gone now, I went to try CS:S again. However, I still got the same message. I rebooted, and got the same message when I started CS:S again. I ran a scan on my comp and Norton said everything is fine. I updated virus definations, Norton still says everything is ok when I run the scan. But as soon as I start CS:S... 'auto protect has blocked a trojan horse, you computer is secure.'

I'm sorry I don't have a concrete question to ask, but I would like to play CS:S, and it wont let me. What should I do?

PS: I bought CS:S legally from steam