The rogue anti-virus software "AV Security Suite" installed itself via Facebook on my PC via an apparently nested PDF file on Facebook, and installed and executed 3 randomly-named EXE files in the process within subdirectories of C:\Documents And Settings\ .
NIS/SONAR missed the malicious nature of these EXE files as they launched, and did not identify them as they were running. This widespread attacker has been dealt with before by NIS with success, but apparently over the holiday weekend the bad guys figured out a way to get ahead of Norton, so I hope new definitions are forthcoming at the earliest.