We'd very much like to write a document with step-by-step instructions on how to make avast skins. However, no such thing exists at the moment, sorry.

However if you'd be willing to do some 'spelunking', you can start with one of the avast default skins and try to replace the bitmaps and regions etc. It's not so hard, in fact.
We're using the ActiveSkin toolkit from SoftShape. You can download the ActiveSkin SDK from here:
http://www.softshape.net/download/activeskin.zip . It includes a pretty nice interactive SkinBuilder in which you can open the avast skin files (*.asws) and quite comfortably edit them.
Of course, the quality of the skin depends greatly on the artwork (which must be prepared in a different program such as Photoshop) but given your original posting it's probably pointless to tell
you this stuff - you know what it's about, right...

Anyway, have fun!
Vlk