Yes, Assembly Virii are popular. I bought The Giant Black Book of Virus Programming by.. that guy... ugh, forget his name, anyway... excellent book, tells you how to write Virii ONLY using Assembler. It's .. well, hard for a C programmer to learn the structure of ASM, but it sure can make a killer virus. He gives you some pretty nifty ideas of what you can do by hijacking system calls, for instance every time you hit a key, make your monitor change colours or something.

Very cool.

There's a version of the book up on the web in PDF format, it's the Tiny Black Book of Virus Programming or something like that, it's free I believe. It's basically most of the stuff from the big book, minus the appendixes and fun tidbits. It's a great read too.

Enjoy,
Tigwyk