On a more serious note, it is reprehensible for Microsoft to have done this. Set Firefox to ask you before installing any add-ons or updates. Get antivirus protection if you don't already have it, a firewall, and install both SpyBot S&D and Ad Aware. Among free antivirus software, Comodo makes an excellent antivirus and firewall suite, and AVG is also good.
To my knowledge, Microsoft didn't install this plugin the normal way, else there's a notice by default. Instead, they basically circumvented Firefox's security, using Windows Update as a gateway through which to do it.
I think even a lawsuit by Mozilla might be in order, not that Microsoft did it intentionally, but that it was at the very least an act of extreme negligence.