Open antivirus is another one if i remember well. But any antivirus or engine will have complicated source code. Clamwin is a fairly easy one compared to popular av products.
If you want to get a basic idea on how a very basic av looks, a basic tutorial like this Making an antivirus engine : Guidelines - Adlice Software can maybe help.
But don't expect you understand/write your own av in a few days if that might be what you are aiming for.