It's a big job to write decent AV software and anyone that's going to be paying you even the minimum hourly rate is going to want some significant differences between that and, say, just using AVG.
You need to understand how viruses work, of course. That much is obvious. That may involve setting up a test machine that can get infected, and it would be useful if it didn't take you out with it.
You need to determine, for each virus, how it infects executables, what to look for to determine that the executable has been infected (just looking for changes to that executable isn't enough because the user may simply have upgraded their software), and you need to do that for all executable filetypes and all filetypes that can contain executable code, including Windows bugs that may or may not be known about.
You need to be aware of viruses that change their patterns to avoid detection and also there are viruses out there that employ anti-detection techniques.
Then you need to determine if the executable can be fixed or if the virus has overwritten critical program code that cannot be restored without reinstalling the program.
Today, my AVG Free reports that it has 1,659,254 definitions installed.
Let's take an extremely conservative assumption that 1 virus = 1 hour's work. Certainly there will be some overlap, work you do for one virus will not need redoing for another virus that works along similar lines. However I reckon it would take more than an hour to analyse even well known viruses.
That's 189 years' work. Continuous work that is, without breaks.
Or 864 years if you work 8 hours/day, 240 days/year.
What's your hourly rate? Current UK minimum wage £5.73 per hour for workers aged 22 years and older (and bear in mind that as a professional programmer you're NOT just going to want the minimum wage). That's £9.5 million. AVG Internet Security Network Edition is £198.58 for a year's subscription, or over 47,800 years.
Great work if you can get it. But what are they getting for their £9.5m/864 year wait that they wouldn't get from AVG, except source code? And I bet you could get the AVG source code if you waved enough cash at them, and that's probably a lot less than £9.5 million. Or that they wouldn't get from ClamWin/Clam?
If you just want to do open source in the AV realm, why not consider contributing to one of the existing open source projects? There are 103 current antivirus projects open at SourceForge, the most popular of which is ClamWin.