[polymorphism code - to hide virus signature]
Randomness is your friend, make your own crypter and make it so fucking random on every compile, that AV reverse engineers kill themselfs (HINT: randomize the crypters sourcecode using perl scripts)
[polymorphism code - to hide virus signature]
I started coding about a year ago, hacking old malware sourcecodes and reading russian boards. Most botnet operators are dumb as fuck, who don't even care about their traces, the ones you see on TV, catched by Microsoft and Brian Krebs. If you have more knowledge you can automatize nearly everything, like creating scripts that rewrite your sourcecode for your crypters so your malware gets undetected again, saving you hard work.
[finding infections on a computer]
Use GMER (http://www.gmer.net/) every now and then when your spider sense is tingling. Srsly, you can't fool GMER, it scans from the deepest possible point in your system, at ring0 and is impossible to fool, there is nothing deeper than ring0 on a usual PC where malware can hide stuff from. I always wondered why other AV vendors don't do it like GMER, it can detect all rootkits. But when a AV can detect everything, who will pay 30$ a year for signature updates...