Will my credit card number be stolen?

I recently entered my credit card number on my computer to subscribe to Sirius. Awhile later, I recieved an alert from Avast saying that I had a trojan on my computer; specifically a "Win32:Trojan-gen {Other}." I'm fairly sure that this Trojan was on my computer before I entered my number, and I'm afraid that someone will have access to my card number now. Any help would be appreciated.