If you're a business user running Windows XP Service Pack (SP) in conjunction with McAfee antivirus software, make sure you read my colleague Ed Bott's blog post before you download McAfee's latest AV definitions.
Other related links:
Microsoft Security Response Center on Twitter: Customers having issues on Windows XP SP3 after updating to McAfee DAT 5958
ZDNet's Larry Dignan: McAfee update derails Kentucky police and a lot of XP machines
Update: Bott has tweeted a link to a remediation tool from McAfee meant to help anyone whose system was affected by the glitch.