Spam Flood Unabated After Bust
AcidAUS writes "Last week's bust of the largest spam operation in the world has had no measurable impact on global spam volumes. The spam gang, known by authorities and security experts as HerbalKing, was responsible for one-third of all spam, the non-profit antispam research group Spamhaus said." The article speculates that the operators of HerbalKing simply passed on to associates the keys to the automated, 35,000-strong botnet, and the spam flow didn't miss a beat.
First, XP has a firewall built in. It's not likely to be "third-party software". Second, firewalls and virus scanners use the same Windows Filtering Platform to do their work. This platform installs boot-time filters that are in effect until the user-mode software is finally up, at which point there's an atomic hand-off. At no time is the system open to any sort of "window of opportunity" like you describe.