jones_supa writes "An interesting bug regarding update dependency calculation has been found in Windows XP. By design, machines using Windows Update retrieve patch information from Microsoft's update servers (or possibly WSUS in a company setting). That patch information contains information about each patch: what software it applies to and, critically, what historic patch or patches the current patch supersedes. Unfortunately, the Windows Update client components used an algorithm with exponential scaling when processing these lists. Each additional superseded patch would double the time taken to process the list. With the operating system now very old, those lists have grown long, sometimes to 40 or more items. On a new machine, that processing appeared to be almost instantaneous. It is now very slow. After starting the system, svchost.exe is chewing up the entire processor, sometimes for an hour or more at a time. Wait long enough after booting and the machine will eventually return to normalcy. Microsoft thought that it had this problem fixed in November's Patch Tuesday update after it culled the supersedence lists. That update didn't appear to fix the problem. The company thought that its December update would also provide a solution, with even more aggressive culling. That didn't seem to help either. For one reason or another, Microsoft's test scenarios for the patches didn't reflect the experience of real Windows XP machines."