I am running XP and I had the same problem. It was the update this week. I had to restore my system to a previous date and I have not downloaded the update. It keeps asking me if I want to. If your running Zonealarm I got this email from them today:
IMPORTANT NOTICE TO ZONEALARM CUSTOMERS
Dear Customer,
Installation of Microsoft Update KB951748 may result in loss of Internet connectivity.
Click here for more information on how to resolve this issue.
We apologize for any inconvenience this may have caused.
Thank you,
ZoneAlarm