Today, Microsoft released a trio of security bulletins covering all currently-supported Windows versions. Users of Windows 2000 SP4 through Windows Vista SP1 (as well as Windows Server 2003 and 2008) need to install the update for the critical Windows kernel vulnerability noted in Security Bulletin MS-09-006. The other two bulletins ( MS09-007 and MS09-008 ) solve important vulnerabilities in SChannel (007) and DNS/WINS Server (008); these bulletins apply to Windows 2000 SP4 through Windows XP and Server 2003 only.
Other updates to look for include the usual updates to the Malicious Software Removal Tool and the Windows Mail junk email filter. If you're on Automatic Updates, follow instructions to reboot if needed after installation. If you prefer to be in charge, don't forget to download and install these as soon as possible.