Backdoor trojan - help

I'm not a computer geek and need to know what to do.

Yesterday I downloaded a file that a customer of mine wanted to show me for a colour refernece. I right clicked and did a scan with my virus scanner (f-secure through shaw cable). It said it was fine so I double clicked on the file.

A warning came up from the virus program that said the computer was infected with a trojan. I don't know the name. It recommended innoculating, but couldn't so it renamed. Right after that another one came up saying the same thing, same procedure.

I then ran a complete scan of my computer and it didn't find anything. Everything was normal today, except when I came back to my computer after dinner there was another virus warning. I saw something about backdoor in the title.

I went through the same procedure as yesterday, virus program renamed the file. How do I know if I'm infected? Should I call someone to come and help me? Is there something simple I can do to figure it out. Should I just reformat. I'm really upset and scared about this. I'm very careful and have never had this happen in all the years I've been using a computer. Please advise.