You don't necessarily have to use assembly (you can if you want to, but good luck learning it). If you know C or C++ it should be good enough. Java probably won't work because you can't directly access the memory, meaning you can't scan for viruses that are already running in memory, making your antivirus app a piece of ****(no offense). (There may be other languages that can directly access memory, but I'm not sure.)
Anyway, whatever language you choose, you'll basically need to have a virus pattern file, which would tell the app that Sasser is a virus and explorer.exe (hopefully) isn't. :) You'll then need a scanner that looks at the memory to see if there are any viruses running, a boot sector scanner to look for boot viruses, and a file scanner to look for infected programs/system files/Word documents (still haven't figured out how you get a virus through a Word document, but apparently some people have managed to, so...), etc. Another good feature to have would be something like Norton's AutoProtect, which runs in the background watching for new viruses. Like I said, you'll need to use a language that can access resources directly.
