Not a large amount I would suspect as we have now pointed him in the direction of a number of them.
However, with XP many are logged in using accounts with administrator privileges. Whilst browsing or collecting email, etc. if you get infected then the malware by default inherits the same permissions that you have for your user account. So if the user account has administrator rights, the malware has administrator rights and can reap havoc. With limited rights the malware can't put files in the system folders, create registry entries, etc. This greatly reduces the potential harm that can be done by an undetected or first day virus, etc. This may have stopped this malware from getting a foothold.
Check out the link to DropMyRights (in my signature below) - Browsing the Web and Reading E-mail Safely as an Administrator. This obviously applies to those NT based OSes that have administrator settings, winNT, win2k, winXP.