Many viruses nowadays actually manage to disable or evade certain antiviruses because people who release the virus code it do to so
Sadly, most of the time this could be avoided if those who are using NT-based versions of Windows didn't infact operate their computers as a user with full Administrator privilages.
If you operate your computer as a restricted user, download an infected file, and then execute the virus, it will be run with your permissions. So it will NOT be able to deactivate your AV software, it will NOT be able to write to the filesystem (except perhaps the C: root and any directories you have write access) and it will NOT be able to deliver a destructive payload. The only viruses that COULD would be the ones that are gained thru a system insecurity, a la MSBlast.
People need to sandbox themselves before any AV software and firewalls can be optimally effective.