In my current company, I’m forced to use IM which is MS Office Communicator 2005. Accidentally (or perhaps my tortured subconsciousness told me do so;), I uninstalled this thing. Unfortunately, re-installation of this application wasn’t so easy as it seemed to be. When I ran installer it exited saying that Communicator could not be installed. That was really strange. After a couple of hours, I’ve finally realized that configuration of my workstation doesn’t really differ from the configuration of a typical WinXP machine, so the problem may be in some service pack or some security update. And that was it. A little creature named “Security Update KB974571” was the reason causing the failure (it was enough to uninstall it and reboot the computer). I wasted about 4 hours of my life which could be spent on coding.