I have one weird error on many of my Windows XP test systems: Internet Explorer can browse to many sites, but not to http://www.microsoft.com.
I’ve tried many things, but the only solution that actually worked was to stop the dnscache service. The idea came from user marcusob11 on http://www.cnet.com.
However, if you encounter such a problem, after disabling the dnscache service make sure you have the latest antivirus installed and run a full scan of the system. On my system I had Conficker 😦