A little less than a year ago, Microsoft announced a critical vulnerability in its Server service. The vulnerability involved specially crafted Remote Procedure Call (RPC) requests, which the Server service would not handle correctly (i.e., drop). The crafter of these requests could use them to execute code of his choice on the server, such as creating or deleting users or changing security policies.
I'd call that a problem. And it wasn't merely potential problem; the exploit for this vulnerability is known world-wide as the Conficker worm, first detected not long after Microsoft's initial security bulletin. Since its inception, Conficker has wreaked havoc on government, business, and home computer systems all over the world, and the investigation to discover its perpetrators is still ongoing.
Our best guess is that the perpetrators are based in the Ukraine, since Variant E of the worm downloads software from a server hosted there. However, they have not yet been fingered, and in the meantime, they continue to control countless infected computers.
Conficker is easy to miss because nothing splashy happens. The most obvious way to tell that you have been infected is that user accounts are locked out or policies have changed, or automatic updates stop running. In the meantime, Conficker has been active on your network or Internet connection, downloading updates for itself and new malware, modifying your registry, establishing restore points, and so on.
There's a Microsoft patch that you can install, and all the major AV companies are able to detach and remove the worm. But despite these facts, Conficker continues to propagate, almost a year after its release, and the perpetrators have still not been found. One of the reasons for this is that Conficker is constantly being updated, and can disable some of the AV solutions before they have a chance to detect and remove it.
There is one very easy way to check to see if you are infected with Conficker: go to this page. If you cannot see all six logos, you may very well have been infected.