This a rare instance of Microsoft releasing a patch on a day other than its regularly scheduled Patch Tuesdays, and indication of how serious the vulnerability was -- and especially how aggressively the hole was being exploited.
The security bulletin with links to various IE version patches is here.
Patch all machines running IE now.
Meanwhile, rival browser-maker Mozilla released 13 patches for its Firefox browser. Nearly half of the patches are labeled critical, including one that closes a JavaScript-vulnerable hole that could render computers prey to malware.
The patched version of Firefox is 3.0.5; this is the only version you should be running.
Firefox 3.0.5 can be downloaded here.
Patch all machines running Firefox now.
Now.