There are some very dangerous Viruses spreading out in the net (Bugbear / Sobig / W32Blast + variants and more) often using known Security leaks/issues within IE/Outlook or Windows
Please use Windowsupdate to get the most recent Security fixes for Windoze/IE/Outlook ! Use Auto/Liveupdate to update your Antivirusprogram/Firewall with the latest Patches/Upgrades/Virus Signature(Pattern) files ! ->> Please do this frequently 1x/week !!!
On infected systems it's possible Firewalls/Antivirusprograms are prevented from running or re-installing - some Viruses/Worms recognize and disable or bypass them !