Why not use some that is alredy exist?

In CalmAV you can create you own definition files. And also create modules for supporting some other applications.

Even if you will create antivirus where can you get definitions from?
And how can you keep it updated?
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oki.oki.......
You can also get look at this http://www.openantivirus.org/


EDIT:
Also read those:
http://www.antionline.com/showthread...hreadid=134048
http://www.antionline.com/showthread...hreadid=113186