There's a new windows XP update that states:
Update for Windows XP (KB887742)
This update addresses an issue that could cause your system to stop responding if certain firewall or antivirus programs are installed. You can help resolve this issue by installing this HTTP.sys update from Microsoft. After you install this item, you may have to restart your computer.
It doesn't say what A/V programs this is for, but I thought I would give everyone a heads up.