SANS InfoCon has been raised to yellow today and for good reason; A  security researcher working for the UK company Computer Terrorism has published a 0-day remote code execution PoC exploit.  This uses an older vulnerability in Internet Explorer versions 5.5 and 6.0 thought to cause a DoS only.  More information is available in the FrSIRT advisory.  Currently the only available countermeasures that I am aware of are:

1. Disable all "active content" (i.e. Javascript and VBScript) in untrusted security zones
2. Use another web browser, at least for the time being (you might not want to switch back)
3. Detect or Block based on signature (there's a Bleeding-edge Snort signature available here)

Be careful out there.