I'm assuming you mean detect the virus, not detect the antivirus :)
Are there any APIs out there you could use that let you pass them a file and they pass back whether or not there's a virus in it? I did a quick google search and I came across a handful of options, but a cursory look didn't produce anything C++ based. However, I'm sure it's just becuase I'm tired and not searching for the right thing.
The point, either way, is that there are SDKs and tools out there for you to use that let you feed in a file, and it spits back whether it's good or not. My guess is that some are free and some cost money.
I think ClamAV/ClamWin have such api's. The key thing is that you need a signature for the virus you are scanning for. All A/V tools such as Clam have large databases of virus signatures that they scan for. I suggest Clam because it is fully open source and free.