writes "According to the latest study by USA Today and Avantgarde, it takes less than 4 minutes for an unpatched Windows XP SP1 system to become part of a botnet. Avantgarde has the statistics in their abstract. Stats of note: Although Macs and PC's got hit with equal opportunity, the XP SP1 machine was hit with 5 LSASS and 4 DCOM exploits while the Mac remained clean. The Linux desktop also was impenetrable, but only was only targeted by 0.26% of all attacks."
See also our story on the survival time for unpatched systems